Attracted to sister?

I posted in here a few days ago, but I was too embarrassed, so I deleted it. But, I'm 17 and I have two families I stay with. My mom and my friends. Me and my other family have been together for 3 years and there's 4 girls and 1 boy. 17F 16F Twins14F 10B. One of the twins, always always runs up and hugs me and is super touchy with me. I think it's weird sometimes but their parents have a 20 year age gap. So I said Oh it must be normal for her to be attracted to older guys. So we kept going and then I began to become attracted to her a little and I didn't even mean to. I'm so close to this family we change in front of eachother at this point. I'm not attracted to anyone else except for her. I really need help getting over her. She calls me her brother and I always call her my sister. I wouldn't date or have sex with this girl at all. I just want to get these feelings gone. Any idea how?

None are biological.