Could be mixed, without papers from a reputable source there is no way to know for sure.
If you are specifically wanting GSDs and have any doubts at all about these particular ones, why are you buying them??
Also please be aware of the issues that raising two puppies together can cause...
I guess to me would it matter if they are purebred or not. Only you know what you want. If my intentions were to breed, show or any other thing that would require a strong good foundation for your project, I would go to a better breeder to start your project. If I wanted a family pet and only a family pet, I would get those pups. If you want a male and a female to breed, I would not get a brother and sister to start your breeding business. I am curious what do the parents look like. That would be a better place to start to see how your pups will look like as an adult. Also, I would wonder if that is the only father to this litter of pups especially if the guy was not careful about keeping the female in his eyesight at all times.
I would be iffy also of being purebred, they could be nothing really stands out much.
I agree that it depends on what you wish for them to do, I would not buy both if I were to buy any as that is not always a good choice. And if you are intent on breeding not to have two that are related that close for sure. As for the fear thing- that picture done seem to point for fear of unknown person or object and is not something you want to see in a breeders pups. Is the Dam on site?
I don't think they are purebreeds either. They look like my mix who is german shep/aussie shep. No papers you can't know for sure. That being said mixes aren't terrible as dogs and I often find their temperate better than their original breeds.