It makes me wonder why every person with incestuous thoughts says, "I don't think I should screw with them", as if they're all sure their sibling reciprocates the incestuous feelings. It's not like you want a relationship, you just want him to screw you. I'm sure there are others out there who are good-looking, though if you're fifteen I'd suggest waiting for a few years before becoming sexually active. Just keep your thoughts in your head, don't act, and maybe they'll go away. Nothing else you can do.
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I hope I live to tell the tale.