It's sometimes a painful realization when the possibility emerges of living in a multi-relationship mode. Because you find yourself caught in what is very much a different world.
My thoughts & suggestion.............
ALL of you spend some time really learning what poly relationships are about. The good, bad, and potentially ugly.
In other words - approach it from a strictly academic perspective. DO NOT personalize it YET ! Don't make it about you and the other wife, about your wife, her husband etc or the current marriages and/or potential relationship.
Keep it strictly about the philosophical and practical aspects of how people successfully live in such a model.
That may remove much of the 'threat' potential, will will keeps emotions at some distance and allow everyone to view and analyze facts for what they are.
This may sound like game playing, but it truly isn't. From what you wrote (?) there is a group of 4 individuals involved here who likely have only had real exposure to what I'll call the 'classic' model.
Before considering adopting a new model it behooves everyone to really know what they are getting into - not just drive by new discovery, NRE etc.
The more even YOU learn, the more you may discover you may not even be up to the challenge ! It's not all fun & games. There's some serious work involved