Attracted to girlfriends sister?

So no rude comments please I love my girlfriend so much but I’m realizing I’m getting more and more attracted to her sister. I can’t help attraction and I feel guilty getting all these sexual thoughts. What do I do? Is it true if I’m attracted to her sister it’s not really love?

Any help appreciated

And to answer some of your questions no she doesn’t know, and I enjoy sex with her so much but can’t help but think what her sister would be like