Marrying sister inlaw

If by sister-in-law is meant, brother's wife, then naturally as long as she is married to his brother, then he cannot marry her- but on his death or if he divorces her, then he can marry her.
The rule is that one cannot marry two sisters at the same time. One cannot have two sisters as wives at the same time. Yes if one dies, or if one is divorced, the you can marry the other one.