If it goes shitty route
Mom will probably confront Hikaru and get mad and get a miscarriage and Hikaru will feel bad and blame himself and be all pitiful and use that to manipulate Haruki into caring for him or at least staying by his side? Then hanabusa will show up but they cant get back together because haruki cant get out of the pity party
if it goes like that I'll re read tadaima okaeri 20x to wash my soul
Additional route: Hikaru (sorry if I'm getting the names messed up), who is so emotionally traumatized that he's a shell of a human being, sees no point in telling her Hikaru initiated it. He has no reason for living. He still loves his family, though, and wants them to be happy. So he lies to the mother and tells her it's his fault so that she won't have to know her son is a monster. He's dead inside, anyway. One less relationship won't change anything.
We've come to a branch in the story. Let's see what routes we have now.
Bad mom route: keeps this a secret to protect her and her son's happiness (being in a stable family with stable income).
Good mom route: Takes responsibility for her son's mistake and make him take responsibility too. (there are lots of ways for this and one of them is divorce and taking responsibility for the child she's pregnant with. There are lots of ways she could butcher this tho and one of them is abortion.) But she has to take responsibility and live with the shame of having a child that raped his step brother repeatedly. Shunning her son is also fucking bad he has to take responsibility too and live with his crime and regret of ruining his family.
Shitty story route: The mom's health deteriorates because of the stress of knowing and keeping this a secret and goes through miscarriage.