How to Form and Use the Plural of ‘Sister-in-Law’: Rules, Examples, and Common Errors

by

English is full of fascinating grammatical structures, and compound nouns—nouns made up of two or more words—are among the most interesting. Family relationship terms like sister-in-law can cause confusion, especially when forming their plural. Do you say sister-in-laws or sisters-in-law? The answer matters for clear, correct communication in social gatherings, formal writing, legal paperwork, or even casual conversations.

This comprehensive guide will help you master the plural form of “sister-in-law.” Whether you are an English learner, teacher, writer, or just someone who wants to be grammatically accurate, this article is for you. We’ll cover definitions, grammatical rules, exceptions, many examples, practical exercises, and advanced notes.

By the end, you’ll confidently use and recognize the correct plural form in any context!

Table of Contents

3. Definition Section

3.1. What is a Compound Noun?

A compound noun is a noun made up of two or more words that function as a single unit of meaning. Compound nouns can be written in three forms:

  • Open compounds: Two separate words (e.g., ice cream)
  • Closed compounds: One word (e.g., toothpaste)
  • Hyphenated compounds: Words linked by hyphens (e.g., mother-in-law)
Type Structure Examples
Open Word + Word ice cream, post office
Closed WordWord toothpaste, bedroom
Hyphenated Word-Word(-Word) mother-in-law, editor-in-chief

3.2. What Does ‘Sister-in-Law’ Mean?

The term sister-in-law refers to:

  • The sister of one’s spouse
  • The wife of one’s sibling
  • The wife of one’s spouse’s sibling

Related terms include brother-in-law and mother-in-law. Each describes a family relationship created by marriage.

3.3. Grammatical Classification

Sister-in-law is a hyphenated compound noun. It is countable, meaning you can have one, two, or more sisters-in-law depending on your family situation.

3.4. Usage Contexts

“Sister-in-law” is common in many contexts:

  • Family trees and genealogies
  • Introductions at social events
  • Legal documents (wills, contracts)
  • Stories and anecdotes

Example sentences:

  • I visited my sister-in-law last weekend.
  • Both of my sisters-in-law live abroad.
  • Her sister-in-law is a lawyer.
  • The will mentioned his sisters-in-law as beneficiaries.

4. Structural Breakdown

4.1. Components of ‘Sister-in-Law’

Let’s analyze the structure:

Component Grammatical Role Meaning
sister Main noun Female sibling or relative by marriage
in Preposition Shows relation via marriage
law Noun Indicates the connection is legal, not biological

4.2. Patterns of Pluralization in Compound Nouns

There are several patterns when pluralizing compound nouns:

  • Pluralizing the principal noun: e.g., mothers-in-law
  • Pluralizing the last word: e.g., passers-by (not passer-bys)
  • Irregular forms: Some compounds have unique plurals

4.3. Pluralization of Hyphenated Family Terms

For hyphenated family terms, the rule is to pluralize the principal noun (the family member), not the ending.

Singular Plural Example Sentence
mother-in-law mothers-in-law My mothers-in-law are friends.
brother-in-law brothers-in-law He has three brothers-in-law.
father-in-law fathers-in-law Their fathers-in-law met yesterday.
sister-in-law sisters-in-law My sisters-in-law are visiting.
son-in-law sons-in-law She has two sons-in-law.
daughter-in-law daughters-in-law Both daughters-in-law are teachers.

4.4. Step-by-Step Guide to Pluralizing ‘Sister-in-Law’

  1. Identify the main noun: “sister” is the word being pluralized.
  2. Add “s” to the main noun: “sisters”
  3. Keep the rest unchanged: “in-law” remains the same.

Example:

  • Step 1: sister-in-law
  • Step 2: sisters-in-law
  • Step 3: sisters-in-law

5. Types or Categories

5.1. Different Meanings of ‘Sisters-in-Law’

The plural sisters-in-law can refer to different family relationships:

  • Multiple sisters of your spouse
  • Multiple wives of your siblings
  • Both types together
Scenario Who they are Example
Your spouse has two sisters Both are your sisters-in-law I have two sisters-in-law (my wife’s sisters).
You have two brothers, both married Their wives are your sisters-in-law My brothers’ wives are my sisters-in-law.
Both situations apply Wives of siblings + siblings of spouse Four sisters-in-law: two from my family, two from my spouse’s.

5.2. Regional and Cultural Variations

Usage can differ regionally:

  • British and American English: Both use “sisters-in-law” as the standard plural.
  • Indian English: May use “co-sister” for a different relationship, but “sisters-in-law” for the standard plural.
  • Formal vs. informal usage: In informal speech, some may say “sister-in-laws” (not standard).

6. Examples Section

6.1. Basic Pluralization Examples

Here are 10 simple sentences using sisters-in-law:

  1. Both of my sisters-in-law are teachers.
  2. I invited my sisters-in-law to the party.
  3. My sisters-in-law live in different cities.
  4. All of her sisters-in-law enjoy gardening.
  5. We went shopping with our sisters-in-law.
  6. Her sisters-in-law are very friendly.
  7. He has three sisters-in-law.
  8. The sisters-in-law met for lunch.
  9. I bought gifts for my sisters-in-law.
  10. My sisters-in-law and I went on vacation.

6.2. Contextual Examples

10 sentences using sisters-in-law in varied contexts:

  • During the wedding, all my sisters-in-law wore matching dresses.
  • The contract named both sisters-in-law as beneficiaries.
  • My sisters-in-law helped organize the family reunion.
  • She called her sisters-in-law to share the good news.
  • My sisters-in-law are attending the conference this weekend.
  • He introduced his sisters-in-law to his colleagues.
  • Both sisters-in-law are fluent in French.
  • The sisters-in-law started a business together.
  • Her sisters-in-law support her through tough times.
  • The painting was a gift from my sisters-in-law.

6.3. Comparative Examples

Contrast between singular and plural forms:

  • My sister-in-law is a doctor. / My sisters-in-law are doctors.
  • Each sister-in-law brought a different dish. / The sisters-in-law brought many dishes.
  • She called her sister-in-law. / She called both sisters-in-law.
  • I have one sister-in-law. / I have two sisters-in-law.
  • My sister-in-law lives nearby. / My sisters-in-law live in different countries.
Singular Plural Context
My sister-in-law is here. My sisters-in-law are here. Visiting
She likes her sister-in-law. She likes her sisters-in-law. Relationships
I have a sister-in-law. I have sisters-in-law. Family
His sister-in-law called. His sisters-in-law called. Communication
This is my sister-in-law. These are my sisters-in-law. Introduction

6.4. Complex Sentences and Extended Contexts

5-8 complex sentences:

  • Both of my sisters-in-law, who are twins, attended my graduation ceremony.
  • My sisters-in-law, each with a unique talent, contributed to the family celebration.
  • After the holidays, my sisters-in-law sent thank-you notes to everyone in the family.
  • Because my sisters-in-law live overseas, we only see them once a year.
  • My sisters-in-law’s opinions are always valued at our family meetings.
  • Despite their differences, my sisters-in-law get along very well.
  • Both sisters-in-law, along with their children, visited us last summer.
  • During the meeting, my sisters-in-law shared their experiences about moving to a new country.

6.5. Other Family Compound Nouns

Singular Plural Example 1 Example 2
brother-in-law brothers-in-law I have two brothers-in-law. Both brothers-in-law are engineers.
mother-in-law mothers-in-law Their mothers-in-law are friends. She respects her mothers-in-law.
father-in-law fathers-in-law The fathers-in-law watched the game together. His fathers-in-law are retired.
daughter-in-law daughters-in-law Both daughters-in-law are teachers. The daughters-in-law prepared dinner.
son-in-law sons-in-law My sons-in-law are very supportive. She has three sons-in-law.

6.6. Dialogue Samples

Here are short dialogues using both singular and plural forms:

  • A: Who is coming to dinner?
    B: My sisters-in-law are joining us tonight.
  • A: Do you get along with your sister-in-law?
    B: Yes, but I have two sisters-in-law, and they’re both wonderful!
  • A: Are your sisters-in-law attending the wedding?
    B: Yes, both of them will be there.

6.7. Summary Table of 20+ Examples

# Example Sentence
1 My sisters-in-law are doctors.
2 We traveled with our sisters-in-law last year.
3 Her sisters-in-law live in Canada.
4 I bought gifts for my sisters-in-law.
5 His sisters-in-law are always helpful.
6 My sisters-in-law and I went to a concert.
7 All sisters-in-law were invited to the dinner.
8 She spoke with both of her sisters-in-law.
9 The sisters-in-law organized a surprise party.
10 I enjoy spending time with my sisters-in-law.
11 My sisters-in-law helped me move.
12 Both sisters-in-law are architects.
13 Her sisters-in-law came from different countries.
14 Our sisters-in-law joined us for the holiday meal.
15 The sisters-in-law brought dessert.
16 My sisters-in-law’s advice is always appreciated.
17 She often meets her sisters-in-law for coffee.
18 All three sisters-in-law attended the meeting.
19 The sisters-in-law share a great bond.
20 He introduced his sisters-in-law at the event.
21 Their sisters-in-law are close friends.
22 My sisters-in-law and brothers-in-law gathered for a photo.

7. Usage Rules

7.1. Pluralization Rule Summary

Rule: Add ‘s’ to the main noun, not at the end.
Correct: sisters-in-law
Incorrect: sister-in-laws

  • One sister-in-law
  • Two sisters-in-law

7.2. Capitalization and Hyphenation

  • Capitalize only at the beginning of a sentence or in titles: Sisters-in-law
  • Hyphens are required in standard English for clarity: sisters-in-law
  • In legal/formal writing, hyphens are always used.

Examples:

  • Sisters-in-law often become close friends.
  • My Sisters-in-law Are Visiting (title format)

7.3. Agreement in Number

The verb must agree with the plural noun:

  • Correct: My sisters-in-law are coming.
  • Incorrect: My sisters-in-law is coming.

Always use plural verbs when referring to more than one sister-in-law.

7.4. Exceptions and Special Cases

  • “Sister-in-laws” sometimes appears in dialects, informal speech, or historical texts, but it is not standard in modern English.
  • Stick with sisters-in-law in writing and formal speech.
  • In casual conversation, people may still say “sister-in-laws,” but it is considered a mistake.

7.5. Usage in Possession

To show possession, add ‘s (singular) or s’ (plural) to the whole plural form:

Form Possessive Example
singular sister-in-law’s My sister-in-law’s car is blue.
plural sisters-in-law’s My sisters-in-law’s opinions differ.

Note: Never write sister’s-in-law or sisters-in-laws’.

8. Common Mistakes

8.1. Incorrect Pluralization

The most frequent error is putting the ‘s’ at the end: sister-in-laws.

Incorrect Form Correct Form Example (Correct)
sister-in-laws sisters-in-law My sisters-in-law are here.
brother-in-laws brothers-in-law His brothers-in-law live abroad.

8.2. Misplaced Apostrophes

Incorrect possessive forms are common:

  • Incorrect: sister’s-in-law, sisters-in-laws’
  • Correct: sister-in-law’s (singular), sisters-in-law’s (plural)

Examples:

  • This is my sister-in-law’s house.
  • These are my sisters-in-law’s books.

8.3. Subject-Verb Agreement Errors

Mistakes can happen with verb agreement:

  • Incorrect: My sisters-in-law is here.
  • Correct: My sisters-in-law are here.

8.4. Confusing Singular and Plural Reference

  • Incorrect: I have two sister-in-law.
  • Correct: I have two sisters-in-law.

8.5. Overgeneralization to Similar Compounds

Some learners apply the rule incorrectly:

  • Incorrect: passer-bys
  • Correct: passers-by

Not all compounds use the same rule, so always check!

9. Practice Exercises

9.1. Fill-in-the-Blank

# Sentence
1 Both of my _________ (sister-in-law) are coming for dinner.
2 Her _________ (sister-in-law) support her.
3 We invited our _________ (sister-in-law) to the party.
4 His two _________ (sister-in-law) are very creative.
5 I enjoy spending time with my _________ (sister-in-law).
6 The _________ (sister-in-law) organized a picnic.
7 She has three _________ (sister-in-law).
8 My _________ (sister-in-law) advice is always helpful.
9 All the _________ (sister-in-law) baked a cake together.
10 The _________ (sister-in-law) arrived early.

9.2. Correction Exercises

  • I have two sister-in-laws.
  • My sisters-in-law’s are very supportive.
  • Both of my sister-in-law live abroad.
  • All the sisters-in-law’s are present.
  • My sisters-in-law is a doctor.

9.3. Identification Exercises

Mark if the plural form is correct or incorrect:

  1. My sisters-in-law are wonderful people.
  2. I have three sister-in-laws.
  3. Her sisters-in-law live in London.
  4. We saw our sister-in-laws at the event.
  5. The sisters-in-law arrived together.

9.4. Sentence Construction

Write your own sentences using “sisters-in-law”:

  1. ___________________________
  2. ___________________________
  3. ___________________________
  4. ___________________________
  5. ___________________________

9.5. Matching Exercise

Singular Plural Definition
mother-in-law a. sisters-in-law c. Wives of your siblings or your spouse’s sisters
sister-in-law b. mothers-in-law d. Female parent of your spouse

9.6. Answers Section

Fill-in-the-Blank:

  1. sisters-in-law
  2. sisters-in-law
  3. sisters-in-law
  4. sisters-in-law
  5. sisters-in-law
  6. sisters-in-law
  7. sisters-in-law
  8. sister-in-law’s
  9. sisters-in-law
  10. sisters-in-law

Correction Exercises:

  1. I have two sisters-in-law.
  2. My sisters-in-law are very supportive.
  3. Both of my sisters-in-law live abroad.
  4. All the sisters-in-law are present.
  5. My sisters-in-law are doctors.

Identification Exercises:

  1. Correct
  2. Incorrect
  3. Correct
  4. Incorrect
  5. Correct

Matching Exercise:

  • mother-in-law → b. mothers-in-law → d. Female parent of your spouse
  • sister-in-law → a. sisters-in-law → c. Wives of your siblings or your spouse’s sisters

10. Advanced Topics

10.1. Pluralization of Other Complex Compound Nouns

Compound Noun Plural Form Example
attorney-at-law attorneys-at-law There are five attorneys-at-law in the firm.
notary public notaries public Notaries public witnessed the signing.
court-martial courts-martial The courts-martial were held last week.
passer-by passers-by Many passers-by stopped to help.
commander-in-chief commanders-in-chief The commanders-in-chief attended the meeting.

10.2. Historical and Archaic Forms

Older texts sometimes used “sister-in-laws” or even “sisters-in-laws,” but modern standard English uses “sisters-in-law.” Literature from the 18th and 19th centuries may show these earlier forms.

Example (archaic): “His sister-in-laws were present at the gathering.”

In legal English, precise terms are crucial. “Sisters-in-law” is used in wills, contracts, and other documents.

Hyphens and the correct plural form are always maintained.

Example excerpt: “All sisters-in-law of the deceased shall be notified within thirty days.”

10.4. Comparative Linguistics

How do other languages pluralize similar terms?

Language Term for Sister-in-Law Plural Form
French belle-sœur belles-sœurs
German Schwägerin Schwägerinnen
Spanish cuñada cuñadas
Hindi भाभी (Bhabhi) / ननद (Nand) भाभियाँ (Bhabhiyan) / ननदें (Nanden)

11. FAQ Section

  1. Is “sister-in-laws” ever correct?
    No. The standard plural is “sisters-in-law.” “Sister-in-laws” is considered incorrect in modern English.
  2. How do you make “sister-in-law” possessive in the plural?
    Add an apostrophe after the plural: sisters-in-law’s (e.g., My sisters-in-law’s opinions differ.)
  3. Can you say “my two sisters-in-law”?
    Yes. This is the correct way to refer to more than one sister-in-law.
  4. Is the plural form the same in British and American English?
    Yes. Both use “sisters-in-law.”
  5. How do you pluralize other in-law terms?
    Pluralize the main noun: mothers-in-law, brothers-in-law, fathers-in-law, sons-in-law, daughters-in-law.
  6. Why is it not “sister-in-laws”?
    Because the noun being pluralized is “sister,” not “law.” Only the main noun takes the plural form.
  7. How do you use “sisters-in-law” in a sentence?
    Example: My sisters-in-law are visiting us this weekend.
  8. What happens if you have more than one type of sister-in-law?
    “Sisters-in-law” can include both your spouse’s sisters and your siblings’ wives. Specify if needed: “My sisters-in-law from my side of the family…”
  9. Is “sisters-in-laws” ever used?
    Occasionally in dialect or error, but it is not standard or acceptable in formal English.
  10. Are hyphens always required in “sisters-in-law”?
    Yes, in standard written English, hyphens are required for clarity.
  11. What is the plural of “ex-sister-in-law”?
    The correct plural is “ex-sisters-in-law.”
  12. How do I refer to multiple sisters-in-law from different sides of the family?
    You can say “my sisters-in-law from my side” and “my sisters-in-law from my spouse’s side” for clarity.

12. Conclusion

Understanding and correctly using the plural form of sister-in-law is essential for clear and accurate communication. The correct form is sisters-in-law, with the “s” added to the main noun, “sister.” Common errors include “sister-in-laws” and misplaced apostrophes. Always use hyphens and ensure subject-verb agreement.

Review the tables and examples provided, complete the practice exercises, and consult this guide whenever you are unsure. Mastery of compound noun plurals like “sisters-in-law” will boost your confidence and precision in both formal and informal English—whether you’re talking about family, writing a story, or preparing a legal document.

Keep practicing, and you’ll soon use these terms with ease and accuracy!

Leave a Comment