21 is a good age for all sorts of mental issues to come to a peak. The breakup may have been a trigger, but the gun was already cocked and loaded. If it weren't her it would have been something else.
The streets of NYC are a living testament to the revolving door of mental heath treatment. When at a mental hospital, you get your drugs with little chance of not taking them. After a bit you get better, but there are side affects. Once you are no longer "A danger to yourself or others" you are released, given your pills and a scrip. The daemons may have stopped howling but the lethargy, living in a world made of molasses, and a host of other side affects, along with no down side to it, leads you to stopping the meds.
The cycle repeats, and repeats, and repeats, until someone gets seriously hurt.
In the classic words from AA, he is not going to want to get better until he hits rock bottom. So far he appears to be far from it with mom and dad ready to make excuses and otherwise bail his but out. So that leaves you, as the only adult in the house, with only one sane option. GET OUT. Get out for your own safety and health. Make sure to tell your parents exactly why, and depending on how uneasy you feel, that you'll likely not come by the house or otherwise visit them with your brother nearby.
God grant me the serenity to accept the things I cannot change, the courage to change the things I can; and the wisdom to know the difference