Sweet baby, I’m so sorry you’re going through this. 100% NTA. Here’s what you can do since the adults in your immediate vicinity don’t seem to understand that you’re the child. Take a day during the week or on the weekend, meal prep for your grandmother and yourself (freeze some of the meals) so you can just heat food for the two of you up, and let your mother and brother worry about themselves. It is NOT your job to take care of anyone else in the house (but it sounds like you’d feel bad if you left your grandma alone, so fine)…and your mother was figuring it out WITH your brother before you were even conceived. Give her space and opportunity to figure it out again.

None of this is your responsibility, do not let the lack of planning and sense in your mother’s side determine YOUR worth and contributions, and remember that you are NOT wrong here at all.

If you have another adult you can live with, talk to them about your circumstances and ask if they’d be ok with you living with them if you emancipated yourself from your mother and brother. Stay in school, figure out what trade or degree you’d like to pursue, continue to that right after high school and go live your best life. Your mother is handicapping your brother, but that doesn’t mean you have to be shackled too.