God is a fictional character created by white people, so that's entirely useless. Also, let's just call a spade a spade here: it's not taboo, it's incest. Whether she is your niece by blood or by marriage, or whatever other establishment, she is still your family. It's disgusting that you would entertain the idea of anything sexual or romantic happening with her, regardless of how close in age she may be to you.
I think TheEvilJester is going easy on you, the gentle soul he is, but you are gross. I will not apologize for feeling that way about you, because what you are doing is entirely inappropriate. Sure, you may not be able to fight the biological urge to be sexually attracted to an attractive female, but you are INSANE to even consider acting on those feelings.
Grow the f*ck up, act like an adult, and get over it. Create distance between you and this person, and go seek guidance from a professional counselor or therapist who is equipped to handle sick people like you.
"Caring is not an advantage."