I regret I never slept with a cousin of mine. There was so much chemistry and caring. It was not a relationship as a girlfriend of course but not nasty sex. There were good feelings but we never did. She is married now but we should have done it and make love outside of our relationships.
So, if you get organize in your heads and understand is basically fun chemistry, go for it. But do not "date her" just benefits.
So much irony here.