Pray and ask God for guidance 

I don't really have an opinion on the cousin thing and I don't think it really changes anything, except that keep in mind if you were to be in a relationship with him, you could even end up losing family over it as people do not take kindly to that.But in the end, he has a girlfriend, it's not your place to ruin that for him. It will pass eventually, distance yourself from him if you have to and distract yourself, do other things with your life, find something else to focus on, you'll move on from him eventually. Just don't go getting in the way of someone else's relationship, for the sake of him and your friend, and yourself too.