Didja ever notice that some celebrities look like they could be related, but they aren't, at all? At our house we call these Muldoons-- as in, "they must be a cousins from some obscure branch of the Muldoon family..." (We picked Muldoon because it is a name we never hear used much these days, and we like it.) Anyway, these are people who could, if paired, easily play family members in a movie, but you never see them cast that way.
Keep an eye out and you'll start to spot them, too.
Tom Hanks and Juliette Lewis (the jailbait character in the remake of Cape Fear)
Ray Walston and Jose Ferrer
Clak Gable and Ernest Borgnine (really, in one scene in Gone with the Wind you could swear Gable becomes Borgnine right before your eyes)