Brothers and Sisters/WMG

Justin is adopted.
Justin is much younger than the other siblings, has a somewhat different personality, and has red colouring in his beard that neither his brothers nor his father have. He's obviously adopted. This explains why Nora has such an large number of children for a middle-class Californian woman - she only had four children, then hastily adopted Justin after some kind of crisis with his parents.
 * Middle class?
 * Red hair is recessive. The parents don't have to have it for it to appear in their children. On top of that, it's not uncommon for women to have unexpected pregnancies in early menopause.

Robert Mc Callister is Sam Seaborn's cousin.
Both are in politics (Sam a staffer, Robert a senator) and both are played by Rob Lowe. So the Mc Callisters may include Sam's mother, Sam's political drive may be familial, and they may just happen to have inherited very similar genes.