MamaCaz is surely right - but don't forget you have a relationship with your granddaughter of 11 years - and plenty of shared history. So does your son. At 11 they have ideas and opinions, needs and wants very different to a toddler so maybe a lot of discussion about her goes on. And maybe your son feels the need to talk about her a lot at home - he may even feel he is "sharing" and being open and honest...
It's natural that the newer mother feels insecure and protective of her own child - who to her will no doubt be the centre of her world. And whilst 33 isn't young, it seems to me that if she's a relatively new mum it's all still very daunting, tagging on to an established family. She may well feel her child is competing for attention (and maybe she feels SHE has to too).
Be gentle on her, I'd say. Try to see her more through your son's eyes and ignore the nastiness. Texts can always be read in different ways too. Try, in future, read them with a smile on your face and in a positive way - with luck you might see something less awful, and in time draw closer to her.
Good luck GrandmaFaye.