Hey if you like your Ex Boyfriend's cousin, and his cousin like you, then go for it! Assuming your both single
If so, then it shouldn't matter that he is related to your ex boyfriend. He's your ex, that ship has sailed, so you shouldn't let him get in the way of this possible relationship.
Who knows, it may be a disaster, BUT, it could also turn out very well, and you won't know until you try.
There's nothing wrong with liking your EX boyfriend's cousin- to directly answer your question ^-^