I keep having fantasies of my SO lesbian sister

I'v been with my SO for two years. We grew up on the same street. He has a gay sister and we all have hanged out together. I find myself thinking about her. Her smile. Her sexy giggle. She just turned 21 and I have invited her out to her first lesbian bar. She laughs and says yes. I would never act on my feelings. After a few days it goes away. I just crush her while she's in my presence. And I masterbate thinking of her when I'm home alone. My SO and his sister doesn't know I am bisexual and I dated a girl years ago for about 2 years. Am I horrible for this? I try to control my thoughts. I haven't seen her since the Family New Years Party