Why Did I Do That Thing

So yesterday my sister got all mad at me because I borrowed her nail polish without asking. And yeah, maybe I should have asked but she goes through stuff so fast it's not like it was a big deal... Then today, I found out she took $20 from my wallet for gas. So now we're both holding grudges and not talking. She keeps saying sorry but doesn't understand why it bothered me so much to use her things without permission. Thing is, when she told me off yesterday over the nail polish? It felt like a punch in the gut. I've always been more protective of my stuff than anyone else though... So yeah now we're both upset and acting childish. And why did I even care so much anyway? My older brother keeps telling us to just talk it out but honestly, I'm still trying to figure this whole situation out in my head.