Does anyone know the origin of this phrase? The earliest example I can find is from the Sumter Daily Item - Apr 11, 1979: “Old timers can rerun just about every lap the Southern without making a mistake, but for some reason the Rebel is kinda like a redheaded stepchild.” This is from the July 1980 issue of Texas Monthly: “My husband beat me like a redheaded stepchild, she told Henry in a flat emotionless voice.” The next example is from a Sept. 7, 1994 in The News: “Unfortunately, Florida by itself is going to probably be treated like a redheaded stepchild.”

Dave doesn’t have this one in the Big List, but Michael Quinion, another great source for questions like this, has discussed it here in World /wide Words.

To save people the trouble of clicking through if they’re just curious about how old the expression is, the earliest cite Quinion found was:

From the day the Republican party came into power the South has been treated like a red-headed stepchild.
The Post-Standard (Syracuse, NY) 29 Jun. 1910.

The citation is also in the OED, entry from 2009.