Struggling with teen

I am a step mom to a 15 year old girl and we are at a loss at this point. Her grades have been terrible for a few years now and we have tried everything: helping her ourselves, tutors, taking her phone, new school, etc. She just doesn’t do her work. She cannot keep her room clean and it’s not just messy, it’s filthy with food and drink that she’s not supposed to bring in there because of this very reason. She lies excessively and manipulates situations to benefit her. She was just caught at school with a vape that she acquired herself and has had for 3 weeks now. She doesn’t seem to care about anything. She is very nonchalant about most things. She even explained to me that she didn’t care when she got caught. Most concerning, she told me about something that happened at her mom’s. She said she accidentally left a knife on the kitchen counter and her toddler brother grabbed it and cut up the playpen. Her baby brother was in this playpen. She explained to me she knows her baby brother could’ve gotten hurt or even killed, but no one knew she left the knife out and she never told them and she doesn’t really care about it. I tried to stay calm as possible and I asked if she has any desires to hurt herself or anyone else. She said no, she just doesn’t care and doesn’t realize what she’s done and the consequences until it’s too late sometimes. We are making an appointment with her doctor for an evaluation because we are very concerned at this point. But until them, it’s stressing me out and I’m really worried about her. Wondering if anyone has any insight.