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AITA for staying with my mom after finding out that my dad and stepmom have been splitting my birthday/Christmas money with my sister?

My (f14) dad and stepmom adopted Maria (9f) like 4 years ago. My grandparents on my dad's side never really liked her and my my dad hasn't let us see them after something happened like 3 years ago.

Every year for our birthdays and Christmas, my grandparents would send us each $150. They sent the check, my dad would cash it, and put the money in a card for us to open with the rest of our presents.

My birthday was 2 weeks ago and I opened the check from my grandparents and saw that it was actually $300. I asked my dad and stepmom why I was only getting half and they said that my grandparents never sent anything for my sister so they saved half to give to her on her birthday and gave her half on Christmas.

I know it was kinda mean but I yelled at my dad, stepmom, and sister for taking my money and I called my mom to pick me up. I haven't talked to any of them since except to tell my dad I don't want to see my stepmom or sister for a while and that I want to live with my mom.

I was telling my friends about it and they said I was kinda being a b__ch to my sister so I wanted to know if I was the a__hole