I have no idea what to do.

So this is pretty much just some need to vent or maybe get advice. My sister has ADHD but that’s only a small part of the story. On several occasions she has disrespected me and my mom saying I hope you die, or burn in hell, etc. my mom will take her phone away as a punishment and that when my sister pulls the I’m going to kill myself anytime she doesn’t get what she wants. (She never does anything to herself) with the screaming all the terrible things and that my mom will give into her and give her her phone back. Ok fine whatever. It just frustrates me that the second I do anything I’m immediately punished but I don’t throw a tantrum when I do I suck it up so my punishment is even worse then hers. On most occasions I’ll accidentally bump into my sister or she’ll do something to me so I’ll do something back 9/10 my mom will yell down from the stairs (my name) stop even though she doesn’t know what happened but when I say something about my sister it’s well o didn’t see it so I can’t say anything to her. I’ll show her straight out marks from where she hit me and it’s still not enough. I know it’s not good to self diagnose but I know for a fact I get anxiety attacks nobody knows it but my chest will start to hurt and it will almost feel like I’m short of breath and I just freak out. Again I don’t tell anyone. On several occasions I get this wave of sadness for no reason it comes from random times and I just feel like crying. Again I don’t tell anyone. I can’t talk to my mom about it because she lags on appointments or just forgets completely I told her once before and she never set me up an appointment. I try to talk to her about the stuff with my sister but she just gets pissed and yells and then I get one of my attacks. It’s not that I hate my mom or my sister it just gets so overwhelming sometimes and I have nobody to talk to.