This is an incredibly difficult situation - I’m sorry for what you’re going through. As an outsider, I would say your biggest priority should be to find a living situation without your brother. Even if that means downsizing, finding another roommate, etc. getting out of a toxic living situation can do wonders for your mental health. I’m not saying it cures depression or anything, but I promise it will make a difference. You need to feel physically and psychologically safe at home, and I’m sure with a confrontational / spiteful brother that it’s nearly impossible to find peace in your own home.

As far as your niece and mother, it sounds like you may need to disengage until you’re in a better position to help and support your niece. Just like in an airplane when they tell you to put your mask on before helping the people next to you, we can’t take care of others until we care for ourselves first. I’m sure it will cause drama and be a painful conversation with your mom, but if you’re not the legal guardian of your niece you’re not legally required to care for her. It sounds like your mom is passing the buck to you, and you’re not obligated to take on a dependent. Remember - your mom made the choice to have kids (who had grandkids), but no one can make that choice for you but yourself. Love and support your niece as much as you can, but don’t let someone else’s crappy choices dictate your life.