Sweetie you arent a bad person, but you are addicted to sex. This is a real problem with obvious real consequences. You need a professional to help you ok?
I'm sure there will be people to disagree with me, but this is something you cant just change unless you can be away from this girl and have people to help you.
This isnt just a situation where we can tell you what to do and you can go do it, this is a chemical imbalance in the brain. Its very real and you need very real help.

You arent in love with your neice, you lust for her. You dont have the feelings for her that you could have with a potential wife or girlfriend. This is someone you know you cant be with, therefore you want her more.
You need to tell a counselor about this.
Here is a link that is similar to your situation... http://www.drphil.com/show/show.jhtm...alpredator.xml