Many people have desires which they know are wrong, including some that they know they will never act on. The problem isn’t so much the desire, which I’m afraid that there’s not much you can do about, but rather what happens after this thought pops into your head. If the moment you have any such thoughts, you push those thoughts away and think about something else, I’m here to say that over time, those thoughts will have less and less of a pull on you. And that would be particularly true if you could form a strong romantic bond with somebody else. But, if you allow yourself to give into these thoughts, even if it’s to beat yourself up for having them, then the thoughts will continue to have a strong impact. So my advice is to understand that you may not be able to stop yourself from such a thought popping into your head, but you have to be prepared to shove it aside as fast as you can. That might require you having a fall back thought. Let’s say there’s some movie star that you find sexually attractive. As soon as the idea of being attracted to your sister arises, shove it aside by concentrating on a fantasy about this other person. Eventually you may not need such a crutch, but if it helps you now, then use it.

  I’ve not really addressed the idea of incest in my response. Your case is a bit different, but it still falls under the heading of incest. You might feel badly if it was your best friend who is heterosexual to whom you were attracted, but it wouldn’t be incest and so you wouldn’t feel quite so badly. What you should understand is that many people find themselves physically attracted to a family member, but because of society’s taboo against incest, they push those thoughts aside. So while I agree with you that you should absolutely not pursue such a relationship, don’t beat yourself up so badly for it as having such feelings for a family member is not as rare as you believe. Your problem is that you are making a mountain of a molehill, and what you have to do is work to bring the problem down to the size. You can keep it under control. And the way to do that is not to worry it to death, but rather to think about it as little as possible.