The family you marry into. Your spouse's mother is your mother-in-law. Your spouse's father is your father-in-law. Your spouse's sister is your sister-in-law. Your spouse's brother is your brother-in-law.
English (my native language). I'm not that interested in mastery, though. I'm taking a broad survey of different languages because I'm interested in linguistics and constructed languages and I want to see how different languages are put together.