rumours can destroy peoples lives,the only way you will know for sure is too confront your wife and your cousin,there is a trust issue here that is obvious,before confronting your wife,do you trust these people who have told you this and have they got real proof,if not this is not something that you can just accuse your wife of,there are no tests to prove if your wife has been sleeping with someone,all you can do is be alert when you gets home.
NO TEST to determine if your wife has had sex in the last 2 months......NONE
As you stated you have heard "rumors" which doesn't make any of this TRUE. Hmmm......sounds like you don't trust your wife. I would give her the benefit of the doubt being she is your WIFE unless you had concrete evidence she has done this; which you don't. I would just ask her if you have these concerns instead of believing what others are saying.