My step mom is forcing me to drive my brother around or taking the car

Hi girlies!

So I turned 18 back in May. I've had my license since I was 16 but not a care. My parents said I needed to save to buy my own car. I planned on getting a job but it just didn't end up happening and I'm working on it right now but my senior year starts in August. My step mom has a second car. She said that I could drive her car around if I want. The only deal is I have to drive my half brother around too. He's 15 and she mainly wanted me to drive him to his therapy appointments since she's a nurse and works 830am-9pm M-F. There's a nurse shortage so she's been working 12 hours a week 5 days a week for the last month. She also said if I could also drive him to a friend's house once and I while. I agreed and I love having a car to use. I typically don't mind driving my brother around but it gets annoying when my boyfriend want to hang out and I have to drive all the way across town to drop him off at therapy or group therapy. But besides that I love him and my step mom. She even let me take her car on a week long road trip. Since school is starting I made sure I could drive the car there and back and my step mom said that was fine but said I needed to take my brother to school too. My brother is 15 and still in middle school. He's in the 8th grade and has been held back TWICE! His school is much farther away from mine and he has to be at school at 730. I have to be at school by 8 which means by the time I get to school it will be time to go to class and I won't be able to hang out with my boyfriend before class and I'll probably miss him being able to walk me to class. And my step mom only wants me to do it because get this. She wants to sleep in. That's it. She wants an extra hour of sleep so she doesn't want to take HER son to school. I told her I want to be able to hang out with my boyfriend before class starts and I can't do that when I have to drive him all the way to his middle school. She started gaslighting me by saying she doesn't ask for much and it's not like I'll be spending any of my money since she puts gas in the car. I said she knows I have no money. She told me if I want to use a car without having to help out with my half brother (HER SON) then I need to gets a job and buy my own car. Even if I had a job how can I get a car on 7.50 an hour? She said either I take my brother to school to or I can't take her car to school and I'll have to take the bus. The school bus to my brothers school doesn't come this far but I suggested he take the city bus because I road the city bus places before I had a car if nobody could give me a ride but she refused to compromise. I called my mom about it and she said it's her car so I have to play by her rules. I don't see how this is fair. She doesn't use the car a whole lot and there are other options for my brother to get to school. I didn't mind driving him to therapy once every other week even if it was annoying but how is this fair?