Basically, as long as you keep it a fantasy, there's nothing wrong with this, and I don't think it necessarily needs any deep dark genesis, like Cthulhu said. You can take it from half the people in this forum, at least, that you are not the only person to have sexual feelings for family members. Things that are forbidden are sexy.
I would chill out on the copping an unwanted feel bit (Unwanted being the operative word. I haven't really figured out my feelings about consensual family sex. If she's into it, and you're all old enough, then I guess there's nothing wrong with it, although I certainly don't think that having babies with your siblings is a good idea) and write your crazy fantasies here.
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