My mom doesn’t want my sister to go back to high school!

So the title says it all! My mom doesn’t want my sister to go back to high school and finish her senior year. I think it ridiculous but I want other people’s opinion. First of all I’m 27 and I have only one sister who’s the one I’m talking about and she’s 17. Last year in November 2017 her best friend 17 was shot in the street 20 times confused by someone else. It was all very messy. The ones who killed this teenager are gang related. They thought this kid knew something and they confused him for someone else. The worst part is my sister was the last person to see her friend because she dropped him off minutes before they shot him. I am guessing they saw my sister and they have involved her in all that mess just because she and this boy were close. The thing is my sister drives this flashy brand new car. We live in a town and I have to say people think we’re rich because of the house we live in and the nice cars we have. You know how small towns are. Anyways my sister went through a lot and she kept getting harassed by cops because they wanted answers she didn’t have. Point is February 2018 my mom took her out of school to be homeschooled. Since then she’s done things I’m not happy about. She used to be a 4.0 student and since last year and all that she started failing her classes, she’s got two tickets, she’s gotten her self into smoking weed (something I’ve never tried in my life) she’s gotten into trying vaporized cigarettes with high nicotine, her attitude has been horrible i feel because she’s just home or all out in about with her friends. My mom won’t do anything about it at all. And this upsets me because I’m seeing every day how my sister is getting worst. She’s a gorgeous girl I wish I could show you to her. She’s talented. She draws beautifu. She’s knows how to skateboard. She’s so athletic. She’s smart. And she’s waisting her life away with the wrong things all because of my mother who’s another story. But the thing is high school is starting in 2 weeks and she’s been telling me she wants to go back to school and finish her high school education which I’m all up for. This makes me so happy because I graduated high school and college as well and I want to help her achieve that as well. Here comes what makes me upset my mom said shes not signing any papers so she can go back because she rather have her in the house which is so ridiculous to me because for 1 it’s me who’s been signing all her school papers and everything and for 2 I don’t think it’s right. So I decided I am going to take this into my own hands and get her back to school. Do you girls think I’m doing the right thing, be honest!