everybody has a past, even you. You must also fall in love with some other women and perhaps having sex with her before you meet your present wife. That's exactly the same thing that happen with your wife. it's just that the girl you make love with at that time was not the wife of her cousin.
So don't take it as she make love with your cousin husband. It's just she a woman making love with a man before she finally met you. cheers!