@GuiltyBoy,
Everybody has fantasies of some sort. There is nothing wrong with fantasies.
But some fantasies are of things that are illegal, immoral, they would hurt people, they make us feel guilty, etc. People who fantasize about murdering their boss - of course they should not do anything to make those fantasies come true.
And you feel you cannot do anything with your cousin (and it may be illegal in your jurisdiction anyway). And even if it's not illegal, if it makes you this uncomfortable, and if she is unwilling, then of course you don't want to act on it.
All of that having been said, I think a part of this is more a simple obsession. E. g. you are fixated on a particular person. Regardless of the familial relationship, it seems much more to be a fixation.
Fixations can be toned down by finding other ways to occupy your time. Go bowling. Take a dog for a walk. Make a model airplane. Read a book. Run a 5K race. Cook a meal. Take a class. Go on a trip. In short, whatever it takes, just do other things.
This does not necessarily mean the fixation will disappear, but you are building up other reserves in your mind - other ideas, other experiences, etc. It doesn't hurt that you are spending time not thinking about this person. Because a lot of these fixations feed on themselves. And, they tend to be more intense to start. It's an infatuation. If you keep reinforcing it, it'll get stronger. If you reinforce other things, the infatuation will fade.
If this fails, seek professional guidance. Obsessive-compulsive behavior is treatable. But see if you can break some of the cycle yourself with simply changing your behavior, and getting and keeping yourself out of reinforcement situations.
