Is it normal for parents to do this?

Is it normal for a parent to tell one child to not end up like their older sibling? I mean I can understand if the sibling is obviously a bad influence, but what if the sibling is well behaved, A or B student, and the only flaw noticeable is some difficulty in getting into a really good university (I won't say which one, but it's like the second or third best university in our state apparently). My mom told me not to be like my older sister, and was completely serious about it. It wasn't the empty words of annoyance like my parents sometimes have. She said "You have to make sure you don't end up like your sister", and when I asked why she got all agitated and was like "Isn't it obvious why?". My sister has never done anything bad from what I know, she's smart, and takes hard and advanced classes too. I could sorta understand if my parents said that to her as one of their forms of trying to motivate her in some way, but my mom only talks about avoiding being like her behind my sister's back. Should I tell my sister? Is this just my parents being too demanding and having too high of standards? My sister gets really upset once in awhile because she feels inadequate, and I think she knows what my mom and dad talk about behind her back. Like one time they were talking about her when we were walking down the stairs and when she walked into the room it got all quiet, but she just acted like she didn't hear anything. I really don't know what to do. Should I talk to our parents for her?