AITA for not wanting another sibling?

I (17m) am the oldest son in my family. I have 4 younger siblings (14f, 8f, 5m and 2f). My mom expects me to watch my siblings when she isn't home. Needless to say, I'm tired of it. I never get to go to parties. Neither do I get to hang out with my friends. In fact, I hardly ever have time for myself. Last time I went on a date with my girlfriend and came home late, my mom got mad at me because I didn't change my youngest sister's diapers.

Today, my mom announced she's going to have another child. It's not that I don't want her to have a child, but I really don't want to watch another sibling. When I told my mom I'm tired of being a "babysitter" and that I won't watch her children, she called me selfish and an asshole for not being happy for her.

My aunt and grandma agrees with me but my dad said I was wrong and that I should be happy for him and my mom. My mom said I "hurt her feelings" and that she's disappointed in me.

But I said it's not my responsibility to watch her children.

That said, AITA?