I experienced this myself and being compared to my sister, cousins, and friends definitely had a negative impact on me. Whenever my sister and I fought, my parents would compare us to some of our cousins, who apparently "got along great." This just made us resent that we couldn't get along like them, and we hated each other more.
I had horrible depression throughout my childhood. Sometimes I was really quiet, which made me look weak and led to bullying. At first, I stood up for myself, which sometimes caused fights (but usually ended better than if I hadn't.) Whenever my parents found out I'd defended myself, they'd punish ME for not just ignoring them. I'd try to explain that there is only so much someone can take, and they'd say, "Your sister/cousin/etc. doesn't have enemies" or "I bet (friend's child) doesn't cuss at people like that." It got to the point where I felt like I deserved to be bullied for some reason.
I hate to sound like I'm blaming all my problems on my family, but I think it really affected my self-esteem, even to this day.
By the way, this question just gave me an idea for a hub. Thanks!