Personally, I don't like the mother. I understand she is human and makes mistakes, but she is continuously forcing her sons into positions they aren't comfortable with while thinking she is being a good parent. From every flashback we've seen, it has always been about her and never about the children, even when she asked for forgiveness, she introduced a completely new man and is expecting their relationship to be better now, expecting all the past mistakes to be forgotten. They're not and we're beginning to see the consequences of it.
This is why therapists are so important in these situations, or if you can't afford it, even doing some basic research. She is also not the only one who is broken. The best solution would have been repairing the relationship first before adding a completely new part into it. Now the loose screw has lost its grip and is fulling apart. The step-father also plays a role in this by forcing himself on the child and not respecting his boundaries. I understand that you do genuinely care for him and are showing him affection, but you are bending his boundaries.
All in all, this is a complete mess of a situation. I hope Jaemin gets some sort of professional support (although I'm very proud of the steps he's made for slowing letting others in and wanting to know more about his brother).
And as the translators have said previously. Mental health is a big issue, especially in the East Asian area. I just love how this manhwa is addressing it, showing a form that is rare in media yet common in real life, and isn't simply being brushed off as mental wellbeing is complicated and can't easily be cured. It's a part of them yet still doesn't describe them. Just 10/10