AITA for refusing to babysit for my siblings anymore and telling my family they can't use me as free childcare and treat me like shit for being an unwanted pregnancy?

I (16M) have four older siblings who are 11+ years older than me. My parents were finished at four kids, my dad got a vasectomy and then six years after it my mom found out she was expecting me. She was 13 weeks when they found out and I was always told if she'd been earlier she would have aborted me. My siblings hated that our parents had another kid. They're all close enough but with me I feel like a stranger and it's not just the age gap. They used to call me "mom and dad's mistake" and "mom and dad's oops" and stuff like that. They'd come home for Christmas and ignore me or make comments about me. They'd buy gifts for each other and nothing for me.

My parents also treated me like a burden. When I was younger they'd call me whiny and ask why I couldn't read my own bedtime stories or why I wanted to sit with them. If I asked for a hug they'd make it such a big deal and say they just wanted me to go away and be quiet in my room. When I had a bad day they were "too busy" to hear me out. Most times they would eat without me too. And sometimes I'd come home and some or all of my siblings would be visiting and they'd have family dinners without me.

Oh, the family dinners they had in restaurants and stuff never included me either.

And once my siblings started having kids I was the default babysitter. If we all vacationed together I was only brought along to watch kids. Sometimes I was left in the hotel or place we were staying alone and they'd have family time. My siblings would just drop off their kids other times at home and tell me to babysit and my parents would give them the okay. I was never asked.

I know people will ask if I'm not the bio kid of one of my parents but I'm both their bio kid. 100% confirmed. It's just I came along when they were done and they resent me for it and my siblings never wanted me.

I tried speaking to my parents and my siblings about how I felt but I'm brushed aside. I never tried writing a letter but it won't make a difference and I'm not pouring my heart out like that to have them not read the letter or to toss it in the trash or something.

I got so tired of it recently. I've been spending more time at my girlfriends house and I hang out with her family more. I started to stay there whenever I feel like they might need me for babysitting and I have refused when told I needed to. On Saturday they wanted to have a family night out and brought the kids over but I wasn't home. Mom called and told me I needed to come home and babysit. I told her to let everyone know that they can't use me as free childcare and treat me like shit for being an unwanted pregnancy anymore. I said I didn't have any say in it and I'm not letting them do this to me anymore.

Their reaction wasn't pretty but I have stood my ground which pisses them all off.

AITA?