CW, I know it doesn't feel like it right now and it hurts like h*ll, but if they're going to live together there is a bright side. It stops being fantasy for them and starts being reality. Real life sets in rather quickly and they can't keep the masks on 24/7 for each other. They each get to find out what gems they've got. Your H will see what he has thrown his entire family away for. Step back away from it. Say or do nothing that can give the two of them something to build a triangle against you with.
It's hard, I live it everyday. I've also heard it isn't everything wonderful that H thought it would be. Hmmmm, imagine that.
Someone once told me to be the better option. I'm keeping my mouth closed and eyes and ears open.
Detach from it the best you can. We will get through this.