It probably has to do with the fact that she's only 7 weeks old...No reputable breeder would let their yorkies go at that age...
Make sure you keep an eye on her for hypoglycemia. Give her frequent small meals to keep her blood glucose levels up. She's still a baby and should be with her mother, but since she can't be, just keep an eye on her to make sure she's eating.
In a few weeks, they will both be playing with each other like crazy, I'm sure!
Edited to add: Keep an eye on both of them as they are both young and many yorkies can become hypoglycemic. Keep some nutri-cal on hand or karo syrup.