Okay, I don't have stepkids, but I WAS a stepdaughter, and to be honest, I want to gain some perspective on things.
I know I wasn't a perfect kid, but I wasn't really horrible, either. I tried. And I more blame my BD than her. But, when I was 9, my folks had been divorced about a year (I later found out it was because she walked in on him with my future SM, but my mom didn't tell me that until YEARS later.)
I didn't see him unless it was his weekend, anyway, but I'm at my grandparents house and her boss casually mentions that my father had gotten married the previous day. I had NO idea.
So, I call, I say, "Daddy, you got married?" (I had met her all of twice at this point.)
He's like, "Yeah, I wanted to surprise you!"
(Right... that's not traumatic to a kid...)
I asked if he "surprised" her daughter, too (she was a year younger than me), and he said, "No, she was in the wedding."
Naturally, I was HURT, but I kind of internalized it, because I was freaking NINE, and we didn't talk about feelings stuff, he and I.
Over the years, I had a feeling she was being shoved in front of me, and I think I resented that, but as I've gotten older, I realized he really preferred her because she played sports and I wasn't into sports; I was a socially awkward, bookish klutz with scoliosis. Though, she was a bully, and her mother, grandmother and sisters made it pretty clear I was just in the way and probably shouldn't be there... Why was I there?
I guess it's hard to have closure with it, because they are both gone now, but I guess I'm trying to wonder was there something I could have done differently that I wouldn't have been resented? Or did they just want her ahead of me?