This isn't necessarily true.. now I'm going to say of course this is coming from a biased side as I'm the "favored" one and have a brother who resents my success.
Factor one: he's adopted, I'm not. My family and myself never ever used this against him or anything. Really I never even think about him being adopted, especially growing up I didn't. It was always known, because my parents wanted to do things right and brought us all to family therapy since a very young and wanted to be honest and open here so it wouldn't be a secret or have issues. But, he definitely feels separate because of this and it plays in our family dynamic when we have issues.
My brother had a lot of behavioral issues since a young child. Was eventually diagnosed as bipolar. Again, therapy was a constant for all of us growing up. There was medication too which helped some. He stopped therapy and medication once he left the house.
My childhood home was chaotic. My brother was very verbally abusive to everyone. He would yell for hours at a time. I was forced to apologize for whatever. We all walked on eggshells. Also, he sexually abused me a few times.
He didn't get better with age. He's still verbally abusive to my parents. I learned to hang up the phone and don't live nearby. He does this to others too.
My parents paid for many years of college. Dropped out. Community college. Dropped out. Technical training. Nothing lasted, usually because of conflict with coworkers.
He resents my success. I did well in school. I have a very high paying career. My parents have had to help him with tens of thousands in debt.
My parents kinda neglected me growing up because they were so focused on his issues. I have been anorexic twice, struggled with substance abuse, have cptsd and classic PTSD from being in an abusive relationship.
But I managed to succeed professionally (also mentally much better these days with lots of therapy in my adulthood).
And my brother resents me for my success relative to what he's comparing me as, and that no one really likes being around him and thus favor me.
I wouldn't say it's always the parents fault.