Got company. $100 missing. Here's what I've done.

[Shoe]

If you are confident he took it, then straight up confront him. Obviously, don't resort to any type of violence. This should definitely not cause any stress between you and your sister/brother, unless they feel like your brothers kid has a right to steal from you for some reason.

Of course if you cannot prove he took it, you need to be ready to eat your words if proven otherwise.