A child who is obviously not your own, a child who is treated worse than other children in the family

Adj. Something that is expected to have a long lifespan by virtue of having existed for a long time, based on the lindy effect, named for the New York restaurant Lindy's, coined by Albert Goldman and popularized by Nassim Taleb.

With red hair being rare, a child born to non red headed parents was often assumed to be the child of an affair. Thus was treated badly, usually in the form of beatings.

A person or group treated without the favor of birthright.

A stepchild might be singled out for abuse. But a red-headed stepchild (who presumably looks like his or her absent birth-parent) might be abused even more because he or she is so obviously different from the other children.

an unwanted, (ashamed of) child. Less than desired child

Treated with dislike, distaste, much the same as a "second class citizen." Less important. Ostracized, at times.
(If the stepchild had red hair, and the parent didn't, people would assume that the parent was the step-parent. This is obviously an ignorant assumption, since the gene for red hair is not a "dominate" gene, but a "recessive" gene -- it would skip generations before showing up again.)