I am leaving for college because of this, why do I feel like this?

Hi, I am a 17-year-old autistic girl who recently graduated from high school. I live with my parents, my older brother, my younger brother, my sister, and her two toddlers in a three-bedroom basement apartment. My teen years have been horrible as i’ve been dealing with mental health struggles like asd, depression, anxiety, and all of that jazz. However, this isn’t what I came here to talk about, I want to talk about what's been happening in my house as of late because I genuinely don’t know what to do. My sister, (29) has been struggling with drug abuse since she was in her teens. She came clean however, and has been for the past 6 years. But, after countless screaming matches with my mom and her self-isolation within the family to help get her clean, she is using again. She decided to leave her two kids, one boy (3) and a girl (1) the night of my graduation party. She told us she would return the next morning. The morning arrived and she wasn't there. A couple of days turned into almost a month and a half and we became extremely worried. She ghosted us or left us short answers such as “I'm on drugs again” or “I need rehab”. This has all fallen on me because my parents work, my brother works, and my little brother goes to football practice every day. This leaves me to take care of the kids during the day. For context, I also have a job, and it's been EXTREMELY stressful trying to take care of the kids in the morning/afternoon and then head straight to work in the evenings. I’ve always been the designated babysitter when it came to my sisters' kids, but waking up at 7 a.m. and spending the day with toddlers who fuss and scream all day is draining, especially since I'm only a teenager and have no idea how to properly take care of kids. At the same time, I know they’re going through so much at such an early age. The oldest asks where his mom is at least once a day. My sister hasn’t expressed feelings of wanting to see her kids, so they haven't seen her in weeks. My sister recently texted my Mom and lied about going to rehab. She also only texts us when she needs money. All in all, the few moments of relaxation I get in between working and taking care of babies are almost useless, because instead of spending time doing things I love, I sleep, because i’m too tired do anything else. Luckily for me, I chose a college 12 hours away in a big city, so I don't have to worry about seeing my family. I'm frustrated it has even come to this point of me saying this, but I'm looking forward to move-in day. I can’t stand living here. I would also like to note that my sister has absolutely NO reason for being on drugs again. This might make me sound like an asshole, but it's true. She has two beautiful kids and lives in a home where her phone bill and everything are paid for. When she lived with us, she chose not to work, and she chose to have kids. So I just don't get why she has to seek out drugs like this. This is too long a post and I apologize, but I want to get this off my chest because I genuinely feel so alone right now and I have no one to talk to about this