AITAH for being mad at my stepdad for already getting with another woman?

I [17F] had an okay relationship with my stepdad before he moved out. We spent some really good evenings playing board games, hanging out, etc., and I can’t even remember when he wasn’t there. We did have a slightly strained relationship because he treated me and my [15M] brother, K, differently than his biological kids, and he would easily get mad at me and K, which led to me having a fear of simply grabbing some food from the kitchen when he was there.

A few months ago, he and my mom announced that they were getting a divorce. To be completely honest, I was slightly relieved. He moved out around two months ago, but here is where the problem started. He and my mom divorced around five months ago, and he’s already begun dating someone else- with kids!! My mom only found out because my sister, M [5F], told her that they had met my stepdad’s “friend” and her three kids. My mom didn’t tell me, by the way; I found out because my sister, M, said the same thing to me, and I asked my mom about it.

At this point, I hadn’t talked to my stepdad for over a month- not because I didn’t want to, but because he never contacted me, and I didn’t either. I don’t know if I’m simply overreacting since it’s been a few months since the divorce, or if my feelings are understandable. Please help.