However charming and beautiful she is, your sister-in-law is not available. To tell her your feelings would damage your relationship with her and your relationship with your wife. It could even damage the relationship between the sisters and your relationship with the entire family. Even if your S-I-L kept your secret, this would be so. She could not look at you without knowing that you are betraying her sister.
But you know this. In spite of knowing it, you persist. Instead of looking deeply at yourself and why you won’t totally commit to your marriage, you distract yourself with the excitement of the forbidden. Ultimately, this will destroy the relationship with the wife you say you love.
I think you are asking the wrong question. You say you love these two women but you are not committed to either. One of my best teachers used to say that two relationships are fewer than one. Spreading your feelings over two people means that neither person gets all of you. For some people, this is a way to avoid taking the total emotional risk that is involved in really joining with another.
My advice? Get into counseling and deal with whatever it is that frightens you about really marrying your wife — all the way. Only then should you make a decision about what to say to the two women.
I wish you well.