Well you said it yourself, you don't mind pissing some people off. It's the morally right thing to do, you have sufficient reason to suspect the kid took it, so ask him.
You don't have to be a dick about it....just go to him and tell him to give it back. Say you know he took it. Whether or not you do, if he's as shitty a person as he sounds, he'll probably deny it if you ask, so just say it like you know it.
If I were your friend, and I knew you had reason to believe your cousin took and you didn't say shit, I'd be pretty upset with you more than the kid who took it. Be a good friend, it's not being an asshole if the person you're being an "asshole" to is an asshole.
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