Out of hand
It started out fairly innocent enough. One time, when she was 15 and I was 17, my sister and I were alone at home for the weekend. I woke up to her watching me through a crack in my bedroom door. She was sitting on the floor, tits out, touching herself. I freaked and snapped at her. She was extremely open minded about the whole thing, saying she had lost her virginity shortly before and loved having sex, but she didn't feel comfortable enough with other partners to experiment and that I was the man she'd always had a crush on since she was little. I told her she was completely mad and told her to never bother me with such a thing again. I never told our parents about the incident, even though she actually insisted a lot in the following months (I had a girlfriend back then, actually).
As I would learn in the following years, her sex drive is so intense it's borderline nymphomania. That drove her to try to seduce me many times over the following months. The more she did, the more I felt curious, then aroused. At one point, I even found pictures of her naked under my pillow... and jerked off to them. Finally, during the sad, hungover morning after my girlfriend broke up with me, she crawled into my bed and started kissing me all over. I'm pretty well endowed, so I ceased any resistance when she took my whole erection in her little mouth (she's very petite, I'm much taller and bigger).
She grudgingly agreed to keep it at just sex, because she wanted a genuine relationship. We spent the next few years having a lot of sex, even though we both dated other people on the side. But secretly, I enjoyed sex the most with my sister, mostly because she's as passionate as she is perverted. To avoid pregnancy risks, besides birth control, I took her anal virginity and mostly fucked her that way... and it drives her mad. I love feeling her cum from an assfuck. Not the healthiest sibling relationship, but I figured she'd grow over it as time passed. We still fucked a few times well into adulthood, but did it less and less as we got into serious relationships of our own. Heck, I even fucked her the night she got engaged, but that was the last of it. That was a little over a year ago.
Only a few days before her wedding, the called the whole thing off over an infidelity of her fiancés. Then she came to me and confessed she knew the guy was a swine that cheated on her since day one, but she didn't care. She just called it off to get out of it because she missed sex with me, and wanted to be with me for real. She even said thing such as "I miss your cock" and "you own me, because you're the only guy allowed to fuck my ass and cum inside of it". I'm not currently in a relationship, so I don't mind going back to fucking her for fun, but she's obsessed with me to the point she wants me to marry her and be her man. Whole different story and I don't know if that's a great idea.