Is there a reason why the Yorkie must be brought into your home at six weeks of age? It's downright illegal in many states for puppies to be sold at such a young age; she really should stay with her mother and littermates until eight weeks, if possible.
That said, it isn't really the German Shepherd mix that you have to worry about. She may act out due to some sort of canine version of sibling rivalry, but other than that, she'll be fine. If she does get pushy, here's what to do:
http://www.mastiffonlinecommunity.net/viewtopic.php?t=1664
But what you really have to worry about is the Yorkie. Yorkshire Terriers are fragile enough to begin with, but a tiny puppy will be VERY easy to injure or even unintentionally kill. Your puppy is pretty big compared to the Yorkie, and if she's a rambunctious dog, you'll need to be really careful to avoid accidental injuries.
Here's how to introduce them properly:
http://www.mastiffonlinecommunity.net/viewtopic.php?t=86
As for whether or not she'll mother the Yorkie, who knows! That one's completely unpredictable. It's unlikely, since the German Shepherd mix is just a big puppy herself (and is thus not very likely to be experiencing maternal instincts yet), but it is possible if she has an attentive, caring, finicky nature.