I think that even if she is family, that no other person (man OR woman) should be taking that much of his time away from you and your beautiful family. If you otherwise have a good strong relationship, I think you should set aside a time to talk to him (instead of letting it get you so upset that you blurt it out in anger) and let him know that you feel like your needs to have his attention are not being met but that he is are instead meeting his cousins needs. And that you would really love to be able to spend that time with him instead of his cousin.
How would that work? It is to the point and honest in a kind, loving way. You don't want to attack him, because he will just attack back, even if he is wrong. That is just our human nature. I hope that helps...I've been there done that...and God helped us through a tough time, but now things are glorious!
M.
www.WorkAtHomeUnited.com/mlarkey