(I suppose the argument could be made that Brendan Fehr and David Boreanaz look like brothers in this still from 'Bones'; but during an actual episode of the show, I just don't see it.)
But there's no arguing the case of Conway Jefferson and his son Frank, seen together only in the opening scene for the second TV adaptation of "The Body In The Library", a Miss Marple mystery:
It's always the structure of the nose for me - whether in looking for tele-genetic similarities or in the depiction of historical figures (a major reason why I never fully bought Paul Giamatti as 'John Adams'.) And in the case of the Jefferson pere et fils, they both have that striking Galllic proboscis.
Of course, there's a good reason why - Conway Jefferson was played by Ian Richardson, and in an uncredited turn, his son Miles appeared as Jefferson's doomed son Frank.