Hi and sorry to see you here, but you will find good support.
I don't know whether you should approach your sil or not. I am bad at those type of questions. That's bad they have cut you off, do you think she would be open to listening to you? Maybe an email? Maybe nothing though, maybe its not your place/business anymore....
One other thought was to cancel the MC appt. This would show your H that you have dropped the rope. You can calmly tell him that there will be no MC when there is a third person involved.
Quote:
withdrawing from the relationship would not be consistent with the healing path she has set out for herself."
Um, ok. Wow. No more contact with her, and no more listening to H's stories of work. That's not your place anymore. Be busy, politely tell him you have to go. Keep conversations going only if they involved your daughter. I know this hurts, but you have to let go. It doesn't mean the marriage is over. But right now, it means you have to let go. Down the road, things might change, but protect yourself now.
You have to remove yourself from their world. Seriously. Emotionally withdrawal and work on your 180's and be very selfish. I might have missed it, but how old is your D?