What does your sister mean that his parents look purebred? If his parents look regular black and tan, then I don't think this pup is theirs. Also if I were you, I would research online on all the risks of the pup being removed from litter before 8 weeks old, you may need to socialize him extremely well to avoid dog aggression later on among other stuff...Best of luck!
Dogs have been homed at 6 weeks old for centuries without issues. By six weeks he is old enough to be weaned, so physically, there is no problem. Mentally, well, yes they can still learn from the dam and littermates. But some litters have only one puppy -- they survive. Some litters lose their dam, or have the dam separated -- they make it too. I think the problems people have with puppies homed between 6-8 weeks are mostly due to the cluelessness the current lot of new pet-owners are, and those same problems afflict dogs homed at 8 weeks and above, if the owners have problems.
In any case the pup was homed at Christmas. 6 weeks later, there is no possibility of returning the pup to its litter for more socialization.
The pup can be sable if the sire or dam was, and a patterned sable looks a lot like a traditional black and tan. I have heard experts arguing whether a dog was a patterned sable or a black and tan. Or the pup can have no saddle.
The pup certainly has a lot of the GSD personality and looks in the photos.
Matty1999, Enjoy your puppy! He's cute as a button, and looks like a ton of fun. Train him, spend time with him, PLAY with him. Don't worry too much about WHAT he is, that don't matter at all. Appreciate WHO he is.