Oldest children are the most jealous because they were once the center of their parents' universe. However, they were soon dethroned or rather displaced by younger siblings. This makes oldest children to believe that they weren't enough in their parents' eyes. They view the younger siblings as usurpers to what they believe was rightfully theirs. They see younger siblings as their competitors for parental time, resources, and attention.
Since oldest children are displaced, many become the perfect child in order to be in their parents' graces once again. They become the responsible one, the overachiever, and other positive roles in order to be perceived as positive by their parents. They also use that perfect child role to gain power over their younger siblings. They will show younger siblings how above par they are so they will look better than their younger siblings. They may even use the role of the perfect child to dominate and bend younger siblings to their will.
In this role of perfect child, oldest children can be judgmental of younger siblings because the former feel that the latter have not or can never reach the former's standards. It is not unusual for oldest children to proclaim how much superior they are to their younger siblings. In a way, it is their way to gain revenge for being dethroned by their parents.
Because oldest children are displaced, they feel disposable. They are oftentimes considered by their parents as not needing them as much. So the least attention is paid to the oldest child by parents. As a result of this lack of attention, many oldest children adopt a highly self-congratulatory attitude towards themselves. They give themselves the attention which was not afforded to them by their parents. It is not unusual for oldest children to resort to one-upmanship to assure that no one will ever take attention from them ever again.