Stepparents are stereotyped as treating stepchildren worse than they would their own children. Redheads were once considered undesirable children, because of superstitions about their unusual hair color. Put them together, and you got a mistreated child.

I think. I once wondered about the meaning. But I could be wrong. By the way, I'm from the East coast, and I've heard the expression many times.

--Jez"actually is a redheaded stepchild"zika