My friend thought something was wrong with me for not being sexually attracted to my stepsister

So I got a new step sister from my dads side about 2 years ago and she's 4 years older than me. She's my half sisters daughter so in other words she's my dads biological grandaughter but my dad adopted her as well. She's not in contact with her parents anymore and my dad and I are the only family she has. She's pretty damn beautiful and my friends always ask me why I'm not sexually attracted to her. She's my sibling and I don't think it's right to see her sexually. My friends tell me that it's better for me to fuck my stepsister who I don't share any DNA with rather than a random girl I meet out in public because some people are more genetically similar than siblings (the example they give me is that Prince Charles and Camilla Parker wind up sharing more DNA than Prince Charles and Princess Diana). Even though my stepsister and I don't share biology we are family and we are siblings and I don't think sexual attraction between siblings is okay even if we don't share DNA.<br><br>My family considers my stepsister to be my full sibling and not my half sibling because of how life is unfair and how we were both given shitty parents. I was given a deadbeat mother and she was given a deadbeat mother and shady ass father. my stepsister and I were given shit hand in life so my dad wants us to be close and he doesn't see her as his grandaughter anymore. He wants us to be close and he wants us to have everything we need and not create life long rivalries between us. Even though we're not close in age my dad wants us to have a close sibling relationship which I think is fine because the age gap makes it easier for us to bond. Our age gap is so big that we don't really compete for things and since my stepsister is older than me I'm basically like a little sister to her.