I feel like a creeper for having these thoughts.
Yikes! Why do you feel like a creeper? Please tell me about these thoughts you're having.
I find myself attracted to my half-sister, I am desperate to kiss her.
It isn't appropriate to have sexual thoughts about your half-sister since she is related to you. It would be a good idea to go out and look for someone who isn't part of your family if you are interested in having a relationship. If these thoughts are really troubling you then it would be a good idea to talk to a professional who can help you work through them.
I can't stop wanting her. I think I love her.
That sounds like a really tough situation, but having those kinds of thoughts isn't appropriate. Perhaps you need to spend less time around her and maybe spend more time with girls who are not family members, so you can focus on being in a relationship with someone else.