What I always found amusing is the amount of JWs I know that have a real Christmas feast on Christmas day. They go the whole hog - turkey, brussel sprouts, stuffing, Christmas pudding! When I asked one family once, they said "Well the shops are full of these things at the moment - it's all you can buy so we might as well - we are not celebrating Christmas though" ??????
But I agree with everyone else. Don't be angry with your Mum - she is doing her best - and at least she loves you and her granchildren enough to make her disregard some of her beliefs and get you pressies. I think it's nice of her. As someone has already mentioned, she may be on the brink of seeing the light herself, who knows?