[[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.
* Justin is not that different from his brothers in appearance as compared to Ryan.

[[WMG: Robert [=McCallister=] is [[Series/TheWestWing Sam Seaborn's]] cousin.]]
Both are in politics (Sam a staffer, Robert a senator) and both are played by Rob Lowe. So the McCallisters may include Sam's mother, Sam's political drive may be familial, and they may just happen to have inherited very similar genes.

[[WMG: The Walker house property has uranium deposits]]
William and Nora were prolifically fertile, but the children they raised in that house have an amazing number of reproductive issues. Tommy is outright sterile. Kitty had trouble bringing any pregnancy to term (the fate of her last one is unknown). Justin has slept with who only knows how many women while drunk and probably not using protection, yet seemingly has fathered no children, and Rebecca's pregnancy failed.