To me, this reads very much like one of the “missing reasons” you find in parental estrangement forums, and Amy (surprisingly) has noticed it: the LW left out her reaction and only put the result: her daughter has gone no-contact.

If the mother had said “oh jeez I had no idea what the hell OK I’m not going to get back together, I’ll make sure you never need to interact with stepbrother again”....her daughter probably would not have gone no-contact. Instead, the LW somehow wants them all to be together again. What? Her daughter, together with her rapist?? Huh, LW. Huh.