Tantrums at 11 😫

My younger sister who is 11 is somewhat emotionally regressed due to a traumatic childhood. She's my half sister and her father was emotionally abusive to everyone for the first part of her life. She doesn't see him at all anymore because he was not a good influence. My mother has been not the best at saying no to her and disciplining her so she's a bit spoiled. She sees a therapist to deal with her issues and I have noticed tendencies that aren't typical of her age. She had a high fever this morning so we kept her out of school and the also had an appointment for her therapist today. She was looking forward to it and we addressed that she cannot go just now because she's contagious, and were very clear that it's unfortunate but we'll reschedule as soon as she's better. She stormed up the stairs (her usual) but instead of crying in her room somewhat quietly, she is in the hallway stomping, crying hysterically, and coughing (she used to do that when she was younger and throw up after a while). My older sister and I tried to ignore her (it's for attention but if my mom comes home and she's still doing that, my mom will poorly react and make the situation worse. Mom sucks at dealing with tantrums). My older sister is currently sitting with her and talking to her so she has finally stopped freaking out. I just wish there were a clear cut way to handle her, she's a lot to deal with. Very explosively angry (not aggressive though) whenever things don't go her way. I'm scared she'll cope with self harm down the line (my sister and I did at her age) so we are bringing it up with her therapist. I just hope we're doing the right thing.

**My mom works long hours and has for all of her children because she's a single parent. It's not as though she wants to be somewhat absent, she just doesn't have the time to be as... parental. My older sister and I are both adults and we all coexist to keep our house running (working, cleaning, everything is done by everyone).