It seems that your wife was never okay with you being poly and now she feels she has to do something extreme to make herself happy, similarly to you're considering now. Was this always above board or was there cheating at some point? If there was cheating, is completely reasonable and understandable why your wife wouldn't want the person you cheated with anywhere near your family and why she would feel rather hostile towards them. Only someone incredibly selfish and toxic wouldn't.
But anyway, it seems like she has reached the end of the road and you both have tough decisions to make. I would stay in situations that make me the happiest. It seems your OSO is the one who does that.