It is nearly impossible not to compare one child to another. Comparisons happen naturally, however, some parents chip away at their children's self-esteem without even being aware they are doing it. Comparisons not only happen at home but in the classroom. When a teacher is already familiar with an older sibling, he or she has usually has a pre-existing judgment of that predecessor. The younger child may be seen as a clone, for good or for bad, of an older brother or sister.
Comparisons are a breeding ground for sibling rivalry.
They can undermine a child's self-esteem. By forcing higher expectations on a child because their brother or sister has previously met those expectations we set them up for failure and weaken their desire to achieve. You are not important for you. A friend, who has the daunting task of raising a house full of boys, described the differences between her sons. Jordan is so reliant, I expect that he be fully responsible and is able to do a lot of things and do them well.