Is your sister in laws brother your brother in law?

It's brothers-in-law.

Your sister-in-law is either the wife of your brother or the sister of your spouse. Your brother-in-law is either the husband of your sister or the brother of your spouse. So your brother-in-law's...

Your brother-in-law's sister-in-law could be your sister. She may also be the wife of one of your wife's other brothers, and unrelated to you.

At times in the past, and still today in some religious groups, a marriage between a brother-in-law and a sister-in-law was considered to be incest. That is not widely the case today.Since they are...

Your husband's brother is your brother-in-law, but is not related to your sister.