Yeah, there has to he legal documentation (usually a will) that specifies all of that. If it isn't stayed you could still, potentially, get custody but you'd have to jump through a number of legal and if they have other family that wanted to do it than you wouldn't likely stand much of a chance. There are exceptions, but again, it's exceptionally complicated.

If you're friends want you to have custody should anything happen you should talk to them about it and let them know that they need to document it all. I know my brother and his wife have it documented and notorized.