Sorry for your brother... As for the data, instead of searching for lost coins it might be easier (and probably more valuable to you) to recover his music.

Try to contact Microsoft - if he was signing in with his MS account, they might be able to help you.

If that doesn't work, detach hard drive(s) from his PC and get them to someone who knows how to deal with data recovery. If you're lucky and he didn't encrypt the drives it could be very easy to get all data from them.