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.
Myles was the red-head step child of the family, he was never invited to the family reunions.
an unwanted, (ashamed of) child. Less than desired child
Mike was the red headed step child of the family, treated pretty shabbily.