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.)
In the state of Alabama, it is well known that Auburn University will always be the Redheaded Stepchild to the University of Alabama, when pertaining to football and all other things athletic - and chess. RTR!