sister in law definition

Sister-in-law – this refers to the husband’s or wife’s sister, or the wife of your brother or sister. Notice that straight away, this one word covers two completely different relationships, which is why context matters when you use it.

Part of speech:
Noun (singular countable)
Phonetic transcription (IPA):
/ˈsɪs.tɚ.ɪn.lɑː/
CEFR level:
B1
Word frequency:

Pronunciation & Spoken Tips for “Sister-in-law”

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The stress lies on the first syllable only: SIS-ter-in-law. In natural speech, the second element, in, is reduced to an almost non-existent syllable: /ˈsɪs.tɚ.ɪn.lɑ:/. The final aw vowel in /lɔː/ is the same as that in the words “law,” “draw,” and “more,” as spoken in British English, so be sure to pronounce it properly.

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Plural & Possessive: The Forms You Must Know

The plural form of “sister-in-law” falls into the category of grammar rules that surprise students even after intermediate-level studies. It is part of the group of “mother-in-law,” “brother-in-law,” and other similar nouns with irregular compounds. Knowing the reason behind this makes it easy to apply this principle to any such noun. 

Form 

Example 

Singular: sister-in-law 
  His sister-in-law works in architecture.

Plural: sisters-in-law 
  Both of her sisters-in-law flew in for the wedding.

Singular possessive: sister-in-law’s 
  Sarah’s sister-in-law’s apartment is two streets away.

Plural possessive: sisters-in-law’s 
  Both sisters-in-law’s opinions were taken into account.

The Two Meanings of “Sister-in-Law”

It’s a term whose true ambiguity causes problems for advanced students when writing formally or completing official paperwork. The two definitions are equally valid and equally frequent in the language. Study both definitions along with the sample sentences, which will allow you to always utilize and comprehend the term with certainty.

Meaning 1: Your spouse’s sister

This is the more common usage in casual conversation. If you marry someone and their sister becomes part of your family circle, she is your “sister-in-law”.

  He introduced her to his sister-in-law at the airport before she had even taken her coat off.
  She and her sister-in-law have been close friends since long before the marriage, which makes family events genuinely enjoyable.

Meaning 2: Your sibling’s wife or female partner 

When you have a sibling who gets married or establishes a long-term partnership, his or her partner will become your “sister-in-law”. The bond may differ in terms of closeness according to family circumstances, but it remains the same name.

  When her brother got married, she gained a sister-in-law who immediately became one of her closest confidants.
  His sister-in-law, his brother’s wife, had been part of the family for so long that most people assumed she was a blood relative.

If there is any ambiguity in a conversation, native speakers will often specify: “my sister-in-law on my husband’s side” or “my brother’s wife.” You can do the same when the context isn’t obvious.

How Native Speakers Use “Sister-in-law” in Real Contexts

The best learners don’t just study words, they study them in situations. Read through each context below and notice the verb choices, the possessives, and the level of formality. One word, three very different registers.

Family Context

  Her sister-in-law called three times on the morning of the move, not to help but to offer commentary. She likes to talk a lot.
  They made a point of visiting his sister-in-law every few months, knowing she lived alone and that the visits mattered more than anyone said out loud.
  She and her sister-in-law disagree on almost everything, from politics to parenting, and have somehow built a friendship entirely on that friction.
sister in law in a sentence

Social & Everyday Conversation 

  My sister-in-law found the restaurant, booked the table, chose the food, and still somehow ended up being the last person anyone thanked.
  She and her sister-in-law have a shared language for family events: one raised eyebrow means ‘let’s leave in twenty minutes’.
  His sister-in-law sends voice notes instead of texts, which he finds baffling, and she finds completely reasonable.

Legal or Formal Situations

  The power of attorney was granted to his sister-in-law, who lived closest to the care facility and was best placed to act quickly.
  The property dispute involved three siblings and one sister-in-law, which made the legal proceedings considerably more expensive than anticipated.
  Her sister-in-law was listed as next of kin on the medical records, a decision they had made together and never updated.

Mistakes to Avoid While Using the Word “Sister-in-Law”

These are the errors that appear most often in learner essays, emails, and conversation. The plural form is the most common written error. The hyphen issue is the most common in typed messages. Catch these now and you’ll use the word correctly every time.

Incorrect  Correct 

Why?
  His sister-in-laws both attended.
  His sisters-in-law both attended.

The plural “s” goes on “sister,” not at the end. 
  My sister in law called.
  My sister-in-law called.

Both hyphens are required in standard written English. 
  Her sister-in-law are visiting.
  Her sister-in-law is visiting.

One person, singular verb. No exceptions. 
  Both sister-in-law came early.
  Both sisters-in-law came early.

“Both” signals plural. Use the correct plural form. 

Spanish, French, and German Translations of the Word “Sister-in-law”

Examine the following three translations. It will make all three easy to remember and will show you an interesting aspect of how other cultures organize their family language.

Language Word for “Sister-in-Law”
Flag Spanish Spanish   Cuñada
Flag French French   Belle-sœur
Flag German German   Schwägerin