I have intense lustful thoughts about my girlfriend's younger sister
I'm very concerned to hear you're fantasizing about your girlfriend's younger sister. Why do you think of her this way?
She is really hot, I want to hit that!
It's normal to have feelings with a person, but if her sister is still a minor, it could get messy if you and her have anything more than a platonic friendship. I suggest that you avoid her entirely and then you will have a clear mind and focus on your current girlfriend.
She's not underage.
I am relieved that she is not underaged, but it is still morally wrong to pursue her as she is your girlfriend's sister and you may end up hurting a lot of feelings if you act upon your fantasies. I suggest you focus with your current girlfriend and make her happy, or properly break up with her if you are no longer interested with her.