Dating

I care about her, I have a brother-and-sister love with her, I feel sorry to her, but I do not want to be with her. What should I do?

I have special relationship with a girl. Our families are friends for three generations, and both have three twins - six kids, one deceased on both side. She is the first in her family and her younger twin brother died, I am the second and my elder twin sister died. That girl is only minutes elder than me, so we became like brother-and-sister, and many people think we are siblings.
Years later, also days ago, we went to trip with some friends. I lived in a two-bedroom suite with her. One night we all got drunk, and I wanted to undress and have sex with her. I said "your body is beautiful" and "I want to fuck you". She was scared at first, but told me she agreed but I "need to take a condom" when I pushed her on kitchen table.
Then we had sex. I choked her neck, spanked her butt, and pulled her hair. She was quite cooperating, and we slept together after that. Next morning she told me she agreed to have sex with me because she loves me deeply, even never had sex with her ex boyfriends, and does not want to be brother-and-sister anymore.
She also told me not to worry, since the sex was with her own consent. But I still want to maintain this brother-and-sister relationship with her. I want to protect her, but not in a boyfriend way. She is a good girl, and treated me like her own younger brother. We helped each other, so that we could conquer the nightmare of losing siblings.
She wants me to consider about our relationship again. Do you have any advice or suggestion? What do you think I should do to refuse her? Or do you think I should refuse her?
I care about her, I have a brother-and-sister love with her, I feel sorry to her, but I do not want to be with her. What should I do?
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