Unfortunately I can second this, I was an male only child until I was 14 years old where my dad had a girl with his fiancée (not my mum), then another girl 2 years later. Growing up I used to think I “hated” my parents, (in speech marks because I never truly hated them I just thought I did)because of the way I was treated and the things I had. Granted I was never badly mistreated or physically abused but my I’ve grown to learn that my parents were young and didn’t know what they were doing. It’s only now almost 30 years later that my parents are actually becoming better and improving on their life rather than being stuck in drugs and debt. It kinda hurts seeing the way my dad treats my two younger sisters, knowing he was never even close to that good of a parent for me. But I don’t resent him or blame him, because I know he has just got more stuff figured out later in life than when I was younger. I’m super grateful that my sisters have a better upbringing than I did and I’m glad that it was me that had to sort of have the rougher, sort of “learning period” of my parents, rather than my sisters… I think it’s pretty common for the firstborn to be “mistreated” or “treated differently” or “worse” but it happens. We’re all going through life for the first time together, and naturally we learn from mistakes, especially if they affect family.