Incestuous thoughts happen... I suppose? So I'll try not to be judgmental.
Distance is the key you need to get over this infatuation you have about her. There is no hope whatsoever in this situation... remind yourself of that. She's married, she has a life.. and more importantly... she's kin, which means your pursuit of her would ostracize you from all those around you.
Go out, get laid... or if you must, try to find someone with the characteristics you happen to like in your cousin. Maybe it's her easy going personality you crave... and not physical intimacy?
In either case... the branch can't grow back into the tree... you need to look a little further from 'home.'
"The weakest soul, knowing its own weakness, and believing this truth that strength can only be developed by effort and practice, will, thus believing, at once begin to exert itself, and, adding effort to effort, patience to patience, and strength to strength, will never cease to develop, and will at last grow divinely strong."
- James Allen