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.
Independent solo polyamorist seeking lover-friends willing to invest in friendship, companionship, and love, but without a need for partnership.
Never confuse commitment with exclusivity, love with ownership, nor sex with intimacy!
For me, it is far better to grasp the Universe as it really is than to persist in delusion, however satisfying and reassuring.
~ Carl Sagan