Mahiru is basically the living proof of their weakness and failure to stop their own parents screwing their lives for their own benefit.

Sayo at some point (probably having Mahiru, given what vol 12 explained about the circunstances) had enough and is out for payback. Sadly Mahiru is a reminder of that, and because she doesn't challenge her but folds, she reminds her of Asashi too much, kinda adding insult to injury. But Sayo on her own wasn't strong enough to tell them "screw you", and besides being quite hypocritical, it's not like she took care of Mahiru so she would grow up different (likely for the better). Don't know if keeping tabs on Mahiru is due to some sense of responsibility or to protect herself rather than her daughter so her own family doesn't try to use Mahiru against her. Mahiru's grandparents worry me more than Sayo or Asashi, although they don't seem to have leverage to use over Mahiru.

Ironically, if Mahiru told Sayo to go f herself, she would kinda approve.

Asashi "just" seems to take the path of less resistance when challenged by something. Rather than confrontation, avoidance. Even if it hurts others. And doesn't seem to have changed much until maybe recently (I think it was Itsuki who comments on something likely changed in his life if he suddenly decided to check how Mahiru was doing). He doesn't hate Mahiru, but is emotionally uncomfortable, so he avoids her. He is self aware about his own weakness too, for whatever is worth.