This topic hits close to home for me.
My DS spends a lot of time with my parents, we live in the same town and they love seeing him and hearing about every little thing he does. He is the first grandchild on both sides.
My DH parents are divorced. DS sees his grandmother (DH mum) and she makes a real effort to come and see us, sends emails, packages. But DH father and his wife have seen DS once since he was born - on Christmas Day. Only because we chose to spend it with them because we knew we wouldn't see them otherwise.
He had his first birthday nearly 2 weeks ago and nothing, not a card or phone call. It hurts even typing this out. I have send cards, photos, emails with photos which I do get replies from but they never initiate. We have asked them to visit and stay, the live an hour away, never bothered.
I think it's really important that your mum not wait to initiate contact with them - encourage her to call them, send cards and photos of her house and herself - this will hopefully encourage them to engage. Tell her not to wait. Some people are just $hit.