Lots of guys fantasize about their sisters in various stages of undress, but won't admit they have feelings like that. BTW, having those feelings of attraction doesn't mean that you're a pervert or sick; it just means you're completely normal. That's OK; the sibling taboo turns me on me too!!!
My blood brother and I are romantically involved, and we enjoy sharing our bodies with each other. It's wonderfully naughty and spicy, and we love pleasing each other. And we agreed to be boyfriend and girlfriend!!! I love him.
But the chances of your sister wanting to do anything sexual with you or wanting to become your girlfriend is pretty much zero, unless she's hinted otherwise.
An open discussion with your sister would be a good idea in your situation. So I'd suggest that you simply talk quietly with her one day when you're both home alone together, or take a long walk around the neighborhood with her. Start off by saying that you've been having romantic fantasies about hugging and kissing her (don't say anything bluntly "sexual" about what your desires might involve).
Tell your sister that she's very beautiful, and that you've always been attracted to her because she's so nice in addition to being beautiful (don't hint that you want her to just strip; the naked stuff would naturally come later if she wanted to start hugging and kissing you a little bit).
If your sister asks what the hell you're suggesting, just tell her that you'd be honored to be her boyfriend. Tell her how you would treat her. Tell her that you'd spend lots of time with her, go on walks with her, take her shopping, help her with chores, help her with homework, and of course hug her and kiss her and hold her. Tell her that you want to be her boyfriend, and that she wouldn't be sorry because you'll treat her wonderfully.
A sexual relationship without a foundation of love and affection tends to makes people feel cheap, based on what my friends are always complaining about. They all have normal jerk boyfriends. That's why I chose my brother to be my boyfriend, because we already love each other and find each other really hot. We also enjoy breaking the brother / sister taboo and sharing passion.
Let your sister think about the possibility of becoming your girlfriend. She might surprise you by admitting that she finds you attractive. Who knows? But I think you'll feel better after admitting your feelings to her, even if she doesn't want to do anything physical with you.
If your sister says "no" right away, that's fine. Tell her that if she changes her mind, then you'll still be interested. After sleeping on your proposal for a few nights, she might start showing some interest in getting romantic with you.
If you're not willing to be your sister's boyfriend, then I think you should leave her alone.
Here's a blog where this guy was really nice to his sister, and it worked out very well for him and his sister -
http://johnnyplussistershannon.blogspot.com
And a few more brother / sister case history blogs -
http://tiffany-and-brother.blogspot.com
http://marissa-and-caleb.blogspot.com
Can you please choose a best answer in a few days and write a few words about what happens if you have a little talk with your sister? I hope she agrees to be your girlfriend!!!!