step-child
A child of a step-parent who assumed the parental role through marriage to one of the child's original (usually biological) parents. Most commonly, the step-parent is the second spouse of one of the child's biological parents. My step-children come to visit their father and me every other weekend.
red-headed stepchild
Someone or something that is deemed less desirable than or inferior to someone or something else, often resulting in mistreatment. Why does management always see the art department as the red-headed stepchild?