Welcome to the forum.
I think that most will tell you that the first thing to need to square away is the difference between you and your wife about the nature of your relationship. Have you even mentioned it to her? If so, what was her reaction? What does she think of her brother's open relationship.
How far have things gone between you and your sister-in-law?
Please check out The Birdcage - an open, friendly Polyamory forum for all parts of New York State
"Listen, or your tongue will make you deaf." - Native American Proverb