It means either your spouse's relatives or people who married your relatives. So: father in-law - your husband's/wife's father sister in-law - your husband's/wife's sister OR your brother's/sister's wife (and I've heard people use it for their spouse's sibling's wife - though that's not technically correct). People will also refer to their spouse's relatives as "my in-laws."

It means that they became your relatives by marriage. Father in law, he bacame a relative because you marryied his. Sister in law, she became my sister because I married her sister.

Father in law is the father of your husband. Sister in law is the sister of your husband. you are a sister in law for the wife of your brother, and so on ...

father in law means husband father sister in law means sister of your husband, I think that ..