I personally think her husband should stand up for her and his family directly to her parents. Those that tend to get all hung up on "moral issues" also tend to be heavily male centered (man is the head of the household, etc). Speak with the attorney, then have him do any and all future communications with her family. If it were me, I wouldn't let them anywhere near the kids unsupervised, if at all (but that's just me).
You said you are in a triad as in all three of you are lovers or is it just you and C?