Well it really depends on how, and why you wanna get back at your cousin in the first place. You could trick them by rehindging the doors in the house so he trys opening it the wrong way, or you could do the penny in the cone trick or etc.
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He doesn't have a cousin...That we know of...
there is no jirachi egg trick, but there is a celebii egg trick in Pokemon crystal
He's his cousin
Look in the trick guide ;)
After you beat the trick master it is not possible to find him in the game again. The trick master is an NPC in Pokémon Ruby, Sapphire and Emerald.