Biologically, I'm the oldest of two (technically, he's my half brother). However my dad remarried later in life and I gained 3 older step-siblings. My experience was 100% growing up as the oldest.
I don't want to get all 'it's free therapy time, boiz!' on y'all but I definitely noticed that my mom treated my brother way better than she treated me. She threw a plate at my head when I asked if she would just sit with me while filling out college applications. My brother took several hundred dollars from her for a deposit to college only to decide to drop out before even starting, and she didn't get mad at him at all. She attended more of his football games in just his last fall than all of cross country/track meets of mine she went to all 6 years of middle/high school combined. I mean, I'm happy for him that she apparently learned how to be a better parent between me and him - because she did start getting nicer to me as she treated him nicer too - but I'm still highkey resentful that my biggest high school bully was my own mother screaming at me and calling me names every night. Meanwhile my dad (I'm his only biological kid) does and has always treated me wonderfully.