Musicians typically leave control of their material to someone they trust, usually family. So when that musician dies, the trusted party takes over legal ownership of everything the musician owned. At the very least, they're able to approve/deny public usage requests, and take legal action when needed.

An artist's work doesn't become public domain when they die, unless they specifically ask for that to happen.

Also, you don't get sued for playing music at a wedding, unless your wedding is in a public area, and open to the public.