So, if your wife and her cousin are like me, the problem is that each wouldn't be comfortable with you having a relationship with her cousin. To me, there is just some level of trust that I wouldn't have if my husband had too close of a relationship with any of my family members... I wouldn't be able to openly complain about the people that other family member is immediately related to anymore, random stories/complaints/secrets about any family members could easily slip out during conversation, etc.
Also, knowing that any of my significant others having sex with ANY family member (sister, cousin, aunt, third cousin, and so on) would make me never have sex with them again. My one weird, possibly extreme limit sexually is that having sexual contact with someone who has had sexual contact with a family member feels like incest to me. lol
I think the goal should be to get the cousin out of your head. It isn't going to happen. EVERYONE has to be comfortable. If you wife isn't terribly opposed to you being with the cousin, then maybe after a while you can try, but most women would find that weird. So, just knowing you have poly tendencies would be the first step. As long as you're happy with where your life is, someone else you're interested in will probably come along when you're least expecting it. If you want to be (and your wife is okay with you being) more proactive, you can join dating sites, try to find local poly groups, and just put yourself out there in general.