As I understand it, based on color genetics in the GSD they can be siblings.
In GSDs white is a recessive masking gene on the E locus. If a pup receives two copies of this gene (e), one from each parent, it will always be white, masking any other color s/he has from a different locus. So one parent could have been white (ee) but not both parents. Or neither parent could have been white but both must have carried one copy of the white gene (e).
Black is on the A locus and in the GSD it is recessive, meaning that both parents need to carry at least one copy of (a) for a pup to have a black coat. One parent could have been solid black (aa) if s/he carried only one (e), or have a white coat if s/he had two copies (ee). Or neither parent could have been black but both non-black parents must have carried one copy of a. For the pup to have a black coat, s/he needs to receive one a from each parent to be (aa), and have no or only one copy of (e).
Both recessives are common with different frequencies in different populations of GSDs. In AKC white GSDs and their puppies can be registered, so (e) is far more common among North American GSDs than those registered with the SV system which started excluding white decades ago. And of course it is ubiquitous in white GSDs bred for the color. Black a is very common in working lines, pet lines, and AKC lines, but has been pushed out in German show lines as they are bred for the red and black saddle pattern.
Color gene experts, please correct if I got this wrong!
Chip v. Dog Pound (border terrier), CGC
Elly v. Rescue (GSD, GSRNE 346), HI(C), CGC, TT
RIP: Feli, Iris, Dago (GSDs), Bubi (spitz), Tanja and Robin (mutts)