I agree, with the last poster, she isnt doing it to be mean, a little thoughtless maybe, but Christmas shopping, trying to get parked, trying to buy something they’d like is horrific, for your own children.
For Grandchildren, worse, as they don’t see them so often, and have lost track of the latest trends.
You said she’s lovey apart from this irritating habit.
I would just accept that you buy their Christmas presents, but tell her what you are buying from her, so that she is clear.
It will make you all happier.
I buy for my mum for my children now, it’s easier for her, and we both hope it’s something that they’d like.