I give you a lot of credit for having the emotional strength to stay friends with a man who you love so much. I hear that you want me to give you words that will convince your friend to stop his relationship with his first cousin.
The first thing you want to ask yourself is, 'Why do I want to tell him what to do with his life?' Are you secretly hoping that if you can get him away from his cousin that he will come back to you? Or is your wish motivated by a disinterested urge to police his morality and help him do what you think is the right thing?
In either case, it isn't your place to tell him how to live his life. This man sounds terrified of intimacy and he chooses women with whom he can't connect (his wife, his first cousin); meanwhile, he refuses a connection with you, a woman who offers herself with open arms.
That's his problem; you can't change him and I don't hear him pounding on my door asking me to help him resolve his intimacy phobia.
So, we're back to you. You've made a decision to remain friendly with a man that you love romantically. Is this a good choice for you? Is this a choice that 's motivated by the secret wish that your faithfulness will one day be rewarded by his returning your love?
If you are secretly hoping for more from this man, then you need to find out more about why you are attached to someone whose unavailable? Was mom or dad unavailable? Are you returning to a familiar emotional place in the hopes of finally winning the love that you lacked as a kid?
If this is your secret wish, you must know that you are doomed to be disappointed since it sounds like this man can't give any more or any better than your parents did. If you can say that you have you truly given up any hope of being with him as anything other than his friend and that you are truly at peace with his friendship, then that 's fine. But don't offer unsolicited advice.
If he asks for your opinion regarding his liaison with his cousin, then tell him what you think. If you want to help him, then focus on his intimacy phobia--that 's the real issue here--not that he has chosen to fool around with his first cousin.
You sound like a lovely, giving woman who deserves to receive back all the love that she gives.