My mo used to do that, ALL the time..
"Why can't you be more like your sister?"
I think when I was younger, it really took a toll on me. I remember being so depressed because I felt inferior to my sister.
Eventually, after a couple of years of treatment that my mom realized what she was doing. By then, my mom stopped comparing me to other people. I told her that I was NOT other people and that no two people should be compared.
My mom now only mentions my sister if I do something like her. Ex., "[Your sister] used to make that face!" or "You're acting a lot like your sister!" It's more of a joking way now rather than a criticism.