If your husband has two sisters, Jane and Sally, then: Jane is your "sister-in-law." Sally is your "sister-in-law." Jane and Sally are your "sisters-in-law." The cat that belongs to Sally (and just Sally - it hates Jane) is your "sister-in-law's cat." The cats (plural) that belong to Jane (and just Jane, they won't go near Sally) are your "sister-in-law's cats." The cats that Jane and Sally share are your "sisters-in-law's cats." And really, that is far too many cats. You may want to think twice about the family you have married into.

The possessive of the singular is "sister-in-law's". The plural is "sisters-in-law". The possessive of the plural is "sisters-in-law's".

sister-in-law's - singular sister-in-laws' - plural It's a strange word, but yes it exists. :)

how to write sisters-in-laws

As far as I know, (1)the main, or most important, noun should be pluralised, except where (2)custom says otherwise (except when it shouldn't). We thus get: Lord Mayor of London --> Lords Mayor of London* by (1), and, sister-in-law --> sisters-in-law** by (1). Moving on to the possessive, we get sisters-in-law's cf. (sisters-in-law)'s. *I actually was not sure about this one and asked the Lord Mayor of Camden about it once. He, a lawyer, prevaricated. **"in-law" can hardly compare in importance to "sister", can it?