Since your daughter is 18, you cannot legally force her to obey you. I wonder if this man could be legally accused of rape if your daughter were considered under the age of consent when she became pregnant or when he began having sex with her. I can't imagine how his wife could sanction your daughter's presence among her children and in her household. This is a very pathological situation.
You need to get help for yourself so you can know how best to help your daughter. Please see a talented therapist who can offer you sound advice and support during these trying times. This is too much for you to figure out alone. Your daughter does need you and will need you even more as this situation moves toward its inevitable meltdown. I'll be thinking of you and hoping for a turn in events.