Why doesn’t she get it? (ADVICE!)

My mum is a single parent to 5 kids, currently living at my house is my younger brother, older brother, his girlfriend, my mum and myself. My younger brother has severe ADHD and ODD and is hard to control. My mum constantly leaves me to look after him (I’m 17 he’s 13) without even telling me, sometimes she doesn’t return until the next day. Now I’m NOT saying she is a bad mother - she’s amazing and tries to do anything to provide us with everything but lately she has been prioritising her friends over us. I tried to explain to her it makes me angry when she does this because some nights I don’t get to sleep until 3am because my brother won’t go to bed, or how he yells at me because she’s not home or I don’t let him have something he wants but yet she continues to leave him with me. I’m in my final year of school (year 12) and I am getting in trouble at school because I’m either too tired and not focused or I haven’t done homework because I’m caring for my brother. I also told her i worry when she doesn’t text me or call to say she won’t be home because I have no way of knowing if she’s okay but she takes it all as a joke.

When I told her I’m sick of her leaving him with me she yelled at me and told me that when she is home I’m just in my room, I don’t think she sees the point that it’s that I’m 17, I should be focusing on school not looking after my brother.

I know it sounds bad and I probably sound like a terrible person but he’s not my child.

Any tips on how to bring this up again? Or anyone else currently/been in this situation?