I want to get with my future sister-in-law
That makes me feel rather uneasy. Why do you want to get with your future sister-in-law?
She has an ass that won't stop, she got it from her mom, but sex with her is all that I want.
Even if your sister-in-law is very pretty, it is wrong to feel strongly attracted to her. You should try to avoid thinking about her.
I can't help it. I'm such a man and sex is all I think about.
Usually people are not attracted to  the family of their partner. You need to stop thinking about this.