liking someone im not supposed to

I know this is going to sound so weird and I don't know how to put this without it sounding that way, but I just need to let it out. When I was younger my parents adopted another kid and yes, at first I did see her as just a "sister" but now I like her. It's not just a phase or anything, I genuinely love her and it's getting really hard to control especially since I see her all the time. And her looks sure don't help. Tbh i'm starting to think she likes me too. She keeps wearing those spaghetti strap shirts around me, showing her cleavage when she knows she has a good body. I keep having inappropriate thoughts about her and I feel like it won't ever stop until she's gone.