I think I am going to steal my cousin's pokemon cards without him knowing
Why would you want to steal your cousin's pokemon cards?
He only knew how to collect pokemon cards and he's a cheater
Even though you think your cousin is a cheater you don't have right to steal his cards. Stealing is never ok.
Who said I'll get caught, it's his own fault for leaving his cards in plain sight. Besides he won't notice they are missing and he will never explain that they are not his.
It sounds like he stole those cards in the first place. Two wrongs don't make a right, so you should be better than him and not steal. He might find out and probably steal them back anyway.