Au contraire, Ginger's mouthing isn't because she doesn't like your Grandmother, it's because she does. The thing for your Grandmother to do is tell her "stop," then give Ginger a chew toy and praise her and pet her when she chews on the toy instead of the Grandmother.
My Shiva is a year old now, but every once in awhile, when she's being extra, over the top affectionate, she'll mouth my hand. Even Bimmer will do it once in a long while. They do grow out of it - mostly.
The only time to watch out for it is if the behaviour seems to be dominance driven. I doubt that's the case with Ginger; at this point, she wants attention and she's trying to bond with everyone in the family. Remember, the only way she knows to do that is to interact in the same way she did with her mother and littermates. Now, what you get to do is teach her how you want her to interact.