Definition of an Evil Uncle
For some reason, the brother (occasionally sister) of the hero’s father has a tendency to be evil. No one is exactly sure why, given that the two were raised in the same environment and it can’t be In The Blood. Maybe simply because of envy, and because the uncle will stand pretty high in the succession row.
This has a tendency to escalate to Cain And Abel. In fact, it almost always does. If he is from a royal family (which he frequently is), he is The Evil Prince. In those cases, he is usually the younger brother, with no children at all, while his older brother usually has at least one. He will usually end up killing his brother, thus triggering the hero’s quest for vengeance. He may try to kill the hero by giving him The Quest for an Impossible Task; this is usually unwise.
A female equivalent is likely to be Wicked Stepmother; the she is unable to inherit herself, but her children can.