You should definately beat the shit out of your cousin, make sure he knows if you ever try shit like that again you'll do more than kick his ass, even if it's an idle threat.

You might also try talking to your mom, figuring out why she'd do crystal, and make sure she knows you care about her and don't want her fucking herself up on that stuff. It's a nasty drug, maybe show her facesofmeth or something. Make sure she realizes you want her to not be a meth-head. No one likes other people to be ashamed of them, this could potentially do alot of good as long as you aren't like, negative. She's probably already feeling kind of shitty if she's doing meth. Might tell her you beat your cousin's ass too, and that if you ever found out she's buying from him again that next time it'll be alot worse for him.

Normally I'd say let people take responsibility for their own actions rather than blaming the dealer, but family selling family meth is just sort-of heartless.

My 2 dolla.