Dealing with Unexpectedly Deceased Husband’s Bitcoin?

Hello, My brother in law unexpectedly passed away, leaving behind my sister, their two year old, and their 4 week old newborn. He had recently gotten into Bitcoin investing (a “To the Moon” mug he had purchased was delivered the day after his death). My sister doesn’t know much about it, I’m trying to help figure it out so we can get the money to her and the kids.

Does this community have any advice on where to start with looking for his accounts and stuff? I was planning on checking his phone for the standard apps (Coinbase, etc), but don’t know any more than that…