Well, first of all, cheating on your wife will definitely not make your relationship better by any means. Telling her down the road that you slept with her sister-in-law will in no way make things better. Your sister-in-law and your wife's brother may be poly, but you are not. Poly is about more then one love with everyone involved fully aware of each others relationships. You are cheating on your wife in a relationship that is already "not very good" at the moment.
It also amazes me that your wife's brother is ok with you cheating behind her back, with his wife. I am sorry to say, but when this does all come out, it will not only destroy her, it will destroy your relationship.