To add on after reading other comments, the mother cannot and shouldn't be forgiven for her actions regardless of whatever happened in the past in the family that resulted in its dysfunction. Yes, it seems as if she is alone and conpletely overwhelmed, but nothing is really understandable about treating her children horribly for things that are beyond their control. Less so when she used her children for things that they cannot protect themselves against, especially given that she had beaten one of them down so harshly that he couldn't even physically protect himself against something he even considered a sin.
i also wondered if the uke's stuttering had anything to do from possible early abuse from his mother. You don't know if she abused him from early causing his stuttering. Very sad and tragic.
The issue is we can't tell if the younger brother sexually abused him just because it's the easy way to think of their having sex.
Ever considered the idea that the older brother somehow managed to convince the youngest brother to have sex because the oldest one couldn't take anymore of the rape and abuse and wanted to feel some love and decided to convince his brother to do it?
This Manga have no POV, meaning there's no way to tell who approached the idea of sex first but given the older brother was the one put in the context of not caring about incest more than his younger brother it's just as likely that he wanted to have sex with him because he wanted to do it with someone who actually loved him to forget this pain induced by the mother.
In retrospect, both the mother and the little brother committed a major crime against the uke. Both committed incest, however I think the reason why we focus on the mother is because she acts in a way that amounts to more of a betrayel of family than the little brother. She locked up the older brother, and then proceeded to verbally, mentally, and sexually abuse him, whereas from a legal standpoint, the younger brother only sexually abused him. The younger brother wasn't the one who essentially made the uke a prisoner. At the very least, there was love between the brothers. The mother calling her child a "pest" and using his existance as the reason for her problems was more than enough for me to know that even if the story was told from her perspective, I would still aide with the brothers.