It's partly stacking, but it's also partly that they've been bred for that exaggerated look, and it's made their legs less stable over time.
If your sister is interested in a more "right angled" shepherd, she should research Shilohs. The woman who's developing the breed started as a GS breeder--she shifted her breeding program partly in reaction to the whole issue of the less stable legs and hip dysplasia.
Even if you don't get one, the whole story of the Shiloh is interesting as an exercise in how people choose what they're breeding for, how they track offspring, etc. It's a good example of how one person does it.