SUPER LONG RANT, I KNOW. BUT PLEASE PLEASE READ TO THE END. IM LOSING MY SANITY

So I'm 20 years old and I have an 11 year old sister whom I babysit every afternoon when she comes home from school. I pick her up, make sure she does her homework, help her with her homework, make sure she does her chores and basically make sure she's done everything before my parents come home from work. I have no problem helping them out because I know they have to work, but what annoys me is that I feel like I was her mom. It's easy for my parents to just assume that my sister will do what she's supposed to, but it's not the same like actually being with her and making sure she does get her stuff done. And it's not just that but whenever there's something school related, since my mom doesn't speak English and my dad is too lazy to read it himself, they'll just ask me to explain to them what the email says. I mean they don't even go online to check her grades. I do that and have to confront my sister on why she's done bad on an assignment. I'm so tired of being the mom figure with my sister. AND to makes matters worse my parents spoil her so much. She is so rude and backtalks and my parents have expressed how tired they are of her attitude, YET they keep buying her stuff, stuff she doesn't need. And then my mom wants to complain to ME about how they're tight with money right now, but yet they keep spoiling her. I'm supposed to understand that they can't buy me anything right now because of money, but yet their 11 year old child, whom I spend more time with than they do, can just be rude to them and still expect to receive whatever she wants. The worse part is that my parents don't even see why this upsets me. They both just ask me why I seem so mad lately and if "they can help me anything". I don't know what to do because if I tell them how I feel, I come off as rude and selfish because I can't "just help them out". I'm not looking for the right way out, I just need some words of advice to help me feel less stressed out about this situation please.