Im an 18 year old man and my cousins a 24 year old man. he is the nicest guy on the planet and we share the same intrests plus hes amazingly hot. he has acctually become so comfortable around me now that he'll change or shower around me. its like every time im neer him i become aroused. we havent done anything. i was wondering if this is normal and should i do anything about it???

You only live one have fun and enjoy it ;)
Forget the cousin aspect for a minute, and think "do I get aroused with other men who I am close to?"