It's hard to tell because it's too short.
As far as we know it there's definitely a chance the two are the other only source of happiness and comfort/love so it's hard to tell if it's the brother raking advantage of the other sibling.
Just because we see the abuse the "weaker one" goes through it doesn't mean that the youngest twin isn't in a bad place himself and the two only can find love between each other because of what happened.
Plus have you ever considered that it might have been the opposite and the older, weakest brother managed to convince his brother (who's equally affected by their mother horrible behaviour just without the rape) have been the one to be convinced into sleeping with his older brother?
I'm not trying to point fingers with who's bad and who's wrong but the one who lost the idea of family is the one being raped by his mother, meaning he could have fell in love first with his younger twin because he doesn't care about family anymore, he just wants the love he gives him that their mother won't provide (in this case I mean the platonic one that because of the bad home situation also turned out to be incestuous) so he wants to forget the rape with some actual good sex made with love from someone he loves and loves him back.
As far as we are concerned even the weakest brother could have been the one convincing the other to do it instead of him taking advantage of his weakest brother.
There's not enough to make assumptions.
Hm... So, in the end the 'good' guy is the brother who takes advantage of the weak mind of his little brother and have sex with him? Because he 'cares' about him... I don't think the abused twin is in the state of mind of consenting freely to have a sexual relationship and his brother is fully aware that the guilt coming from this relationship is torturing his already precarious sanity. So in the end, for me, the mother and the brother are both bad guys here. I have mixed feelings about that story...