- the sister of one's husband or wife
- the wife of one's sibling
- the wife of a sibling of one's husband or wife
Nora is best friends with her SIL (sister in law) and wants her to be the baby's godmother.
An example of sister in law is the sister of your brother.
The plural sister-in-laws is occasionally seen, but this is considered incorrect by most sources; see, for example, , despite being a common construction in American English.