the most disrespectful 13 year old I know 🙄

I have a some what troubled sister & my mom doesn't really know how to handle her & I really want to give some kind of help. I don't normally get along with my mom but seeing how my sisters been acting lately I hope to give at least some kind of helpful tip. Pretty much my younger, 13 year old, sister has just been acting like a straight up bitch. Yesterday before I left for school my sister (who was already at school) texted my mom "I'm going over to Dayshas house because I don't want to see you or Allison so I'm not coming home and I'll see you tomorrow after school" ... we waited for her to come home & she didn't. My mom told her that she had better not otherwise there'd be consequences & her reply was "ok cool bye" According to my mom, she's pissed about something but she doesn't know what. The other day my sister & her friend begging my mom to take them to Taco Bell (which is about 20 minutes away, keeping in mind we live in a town that has plenty of resturants within 5 minutes away) & my mom was going to but my sister kept taking pictures of her & laughing with her friend. My mom noticed so she told her she wasn't taking them to Taco Bell anymore & she had a fit about it. She's told my mom multiple times that her life sucks & says it all my moms fault. I've told my mom she should discipline her by taking something away but every time my sister acts up her response is "I'll do it next time" my mom had asked me to do some snooping to see if I could find anything on social media that might explain her recent behavior & I looked to find an older (only 6 months inactive) twitter account & one of her tweets was "My mom asked if I drink or smoke. I knew she was dumb but I didn't know she was that dumb." I just feel so bad because my sister is starting to become out of control. Last year she got in trouble because my mom had found out she was sneaking out to the garage to have & give oral sex to her boyfriend & for sending him nudes. I know I'm not the parent here, but my mom has never taught us discipline. My dad was the one who would always discipline us, but he passed away when I was 9 & my sister was 3, so most of my life my sister has had no to very little discipline throughout her life. I'm not close to my sister at all. We've never gotten along so I'm unable to talk to her & every time I do I just get a "ok whatever" or "what the hell do you want from me" & then an "ok whatever"