We all have attractions to people that would be inappropriate for us to pursue, for various reasons. It seems to me this is one of those cases. You've acknowledged it to yourself, now let it go. It seems like a crush that can be very pleasant if you simply let it be -- and could become a good friendship -- but will only turn into a big mess if you try to have a sexual relationship with your sister-in-law. I think you're just excited about "going poly" and she is in close proximity. Cast your net wider AFTER you and your wife have some agreement on whether to pursue polyamory.
The world opens up... when you do.
Oh, oh, can't you see? Love is the drug for me. ~Bryan Ferry
"Love is that condition in which another person's happiness is essential to your own." ~Robert Heinlein