I really don't know if I'm AH in this case. I apologize for my bad English.
My(19M) parents divorced when I was 6. My mom got a boyfriend(Josh) and they started living together when I was 9. He had twin daughters, who were 4 when they moved in. Their mother was not in their lives anymore.
My mom took them in as their own. I admit that I was a little jealous because they had her full time and me only 50% of time. I think my mom loved them more too.
I was a quiet kid and spent most of my time in my room when my mom had me. I didn't want any siblings. My mom tried but to no avail.
By the time I was 16, I rarely spent time at my mom's place. And when I went to college I got an apartment, even though my mom lived in the same city as my college.