It's certainly possible. If you are a woman, your sister-in-law's sister-in-law could be you; if either gender, that might be your own sister. Or, it could be someone else who is no blood relation to...
No, your wife's sister-in-law may be your sister. Your wife can also have a sister-in-law who is not related to you; that would be the wife of her brother.
Your sister's sister-in-law is your sister-in-law, but only if your sister's sister-in-law is the wife of your brother. If your sister's sister-in-law is the sister of your sister's husband, then she...
Both can be correct, depending on the situation."Sisters-in-law" is the plural, referring to two or more women who are each your sister-in-law. "Sister-in-law's" is the possessive form, describing a...