<human>: My girlfriend (Roxanne) and I have a happy, committed long-term relationship, going on over 4 years.  I was a forever alone before meeting her and I never thought I'd find someone I connect with nearly as much as I do with her.  We live together and we're practically married.  

Before this relationship, I had no luck with women and getting crushes on any reasonably attractive woman who showed me attention was commonplace.  I figured out that when I eventually realized the feelings weren't mutual, it was best to cut the crush out of my life completely, save myself the heartache and move on.

Back to Roxanne, we click on a physical and emotional level that I didn't think was possible.  I want to spend the rest of my life with her and I don't have any doubts about that.

However, Roxanne has a sister named Marie.  Now I'm very attracted to Roxanne, but I'm very very very physically attracted to Marie.  Add to that Roxanne and Marie are very close and we live one block apart, so they're always at each other's houses.  Which means Marie is always around at our house, at least a few times a week.  Cutting her out of my life is pretty much impossible.

Now Marie and I get along well, but there's no chemistry at all.  My crush is purely physical.  Nothing has ever happened between Marie and I, no inappropriate flirting of any kind or any hint of it.  Nor would I ever go ahead with anything with Marie.  But lately, I've been having sex dreams about her.  I can't help but be very excited when I find her in my house when I come home from work, chatting with Roxanne over dinner.  It doesn't help when she complains about currently being in a bad relationship with no sex life to speak of and then praises me for being a great guy who treats her sister so well.

I can't stop my brain from all these intrusive thoughts about how attracted I am to Marie.  Typically I'd just cut her out of my life, but that can't happen.  What to do?

Could you distill the essence of the text into 1-3 sentences?
<bot>: I'm very physically attracted to my future sister in law.  I can't cut her out of my life because she's close to my girlfriend.  How do I stop thinking about her?