The title of this should speak a lot to you, and if you've clicked it you've come to flame in the comments or you genuinely want to know what I have to say, and whether or not I'm telling you incest is okay-well I'm not.
What I will tell you is that I know exactly what it is you're experiencing. Every once in awhile your sister will come into the living room barely wearing anything, often not wearing a bra and form fitting clothing, and your initial reaction is to be disgusted while trying to tell yourself not to continue to examine her, but you look anyway-but not because you're being a pervert, just because you've said something to give you a legitimate excuse to be looking her down.
Later that night you're laying in bed masturbating and she keeps popping into your mind, and it's both strangely erotic and repulsive to you. "What the hell is wrong with me?" runs through your mind, but the more you're disgusted by yourself the hotter it seems to become. Eventually you begin to embrace these thoughts and it turns into some sort of kinky fantasy, you become more bold in your actions and thinking, you find yourself shamelessly taking peeks when she bends over, but it's still in the back of your mind you're a disgusting freak.
One day you cross the line, you steal some of her underwear and use it as a sexual stimulant. The thought crosses your mind to put them back into her underwear drawer so she could wear them drenched in your cum, the idea is oddly tantalizing, but then reality sinks in and paranoia envelops you. What if you get caught? What if she notices it and knows what it is? What will you do? So you decide to make sure they make it to the washing machine, but then you begin to lose track of how many you've taken and when, you forget if you've put them back or if it's still covered in semen under your mattress, but you're at school or at work, can't get home, what if mom changes your sheets and finds them? Fuck. Fuck. You need to leave work and make sure you took care of the evidence, but you can't, and roll the dice with a lot of stress. But it's fine, you did indeed take care of it.
You find yourself online at places like 4Chan anonymously talking about potential incest and asking for advice, the common suggestion is to just make a move and see what happens, it's stupid, you'd burn in hell, but at the same time you consider it as a viable option. Days continue on and you've shrouded yourself in this fetish, maybe going as far to putting spy cams in the bathroom or bedroom, often with extreme paranoia and with no success, and perhaps maybe you score something and it brings you on an entirely different level of immersion.
Finally you begin making jokes to her about incest, but in a way you could pass it off as your dark sense of humor because you're often involved in "memes" and the like, people know you're a rather twisted person but also harmless, but for some reason the way she responds leaves you unsure anyway, there are a few times you think she might of showed an interest but you just could not know for certain.
You decide to get absolutely shit faced drunk in the night when you're alone, and it's fun and you hang out and watch movies, or maybe a video game. Something happens, your drunk-ass makes inappropriate contact with a part of her body and she immediately responds negatively, recoils and leaves.
She doesn't tell anyone, but you don't talk anymore.
This is because women aren't as deviant as men are, and you just destroyed your relationship with your sibling. This is the likely case for the majority of people in this scenario. If you're not the one she's coming on to, chances are you don't stand a chance in hell at success. Sexuality is an important part of a man's developing mind and body, and if you get stuck focusing on the only female close to you, you'll end up in a position as described in this post.
Don't follow the rabbit down the hole is the moral of the story.