I will say that it is really weird but like I think that his actions might save her, not fix her but maybe finally allow her to truly tackle the problems she has.
He is honestly the person who cares about her the most, her mental health problems are not her fault she was born into a fucked situation and had to suffer greatly.
I hope she can maybe find peace and heal from her trauma and pain, no one deserves to suffer like she has, lil bro is definitely being self sacrificial about all of this but we have seen that is what he wants, he is happy helping others even at his own expense, and that is a result of having received plenty of love growing up and wanting emulate his heroes.
There is nothing wrong with wishing to help others even if you loose from it, we wouldn't say that being a fireman is wrong even if there is a chance you could get injured, so if he chooses to try to help her while knowing what he is getting into then it might be painful but it is his choice, right or wrong is truly subjective in this case.
No one would blame you for giving up for your own sake, but also I don't think it is wrong to truly care so much for someone that you might sacrifice for them, he definitely has to be able to separate his wishes and desires from what she needs too.
If he only selfishly pursues her for his own sake it is only going to hurt everyone, but if he realises she needs help and is not really able to truly deal with everything that comes with life at this point he can possibly be the person she needs in her life.