Nips and Nibbles

I have a German Shepherd bitch (Hope) who is about two years old, and recently bought two little pups (both girls, Nalah and Kitana) who are now nine weeks.

Hope is very maternal with the pups, and often keeps food aside for them and cuddles them to sleep.

Recently I've noticed Hope is nibbling Kitana all over her body, what does this mean? It's only ever Kitana, as she's the more playful one with Hope. Should we be worried about it? She's never hurt the pup, and the pup always goes back for more.

Thanks
Beth, via email