AITA for refusing to get out of the car so my cousin could go shopping instead of me?

About an hour before a planned shopping trip, my sister told me she was going out with my mom. I said I wanted to go too, and my girl cousin also said she wanted to come. My boy cousin originally said “no” when asked if he wanted to go, but apparently later (when I wasn’t there) he changed his mind.

The car my mom wanted to drive only seats 5, which meant she could only take 3 kids. My sister told my boy cousin that he couldn’t come because of that. We asked my mom to take the bigger car so we could all fit, but she refused because she’s “more comfortable” driving the smaller one.

When it was time to leave, we were already in the car (me, my sister, and my girl cousin), and my boy cousin was at the door. My mom started saying, “Can’t he just sit on someone’s lap?” and when told that’s unsafe, she asked me to get out instead so he could go.

I said no — I had already said I wanted to go earlier. Plus, I was going to shop for new shoes before heading off to college. We’d already argued earlier about how many shoes I could get (I wanted 5 now, she said only 3 now and 2 later). My shoes are really worn out and I haven’t bought any in a long time (I know this sounds stuck up, but really, my shoes are really worn, like scratches all over the soles and soles coming off), so I felt like this trip mattered to me. I said if I were to get down, my mom should let me just get the 5 shoes, because disrupting the plans I had, I argued, wasn’t fair.

My sister said what I said was rude, but I refused to get out. In the end, my mom got annoyed, everyone got down from the car, and then my sister said she didn’t want to go anymore. My girl cousin cried, and my sister blamed me for everything.

I do feel bad because technically my sister’s shopping was more urgent and my school starts later, so maybe the “mature” thing would have been to get out and let my cousin go. But I also think it’s unfair to kick me out at the last minute when I had planned to go from the start.

AITA for not getting out of the car?