sorry to hear about your gran spicy
dont pressure your mum into talking if she doesnt want to but just let her know youre there in case she does, sometimes thats enough. when my nan died earlier this year, im not sure how the rest of my family took it to start with cos i was at uni so whereas they all found out in the morning i didnt know til the evening when they came and told me. but we didnt really talk about it, im not sure whether thats cos we didnt wanna admit to the fact or cos we didnt really wanna cry in front of each other but just sitting there as a family knowing that we were all upset about it was enough. im glad my mum made me come home cos to start with i wanted to stay at uni but i know now that wouldnt have helped me, and my dad wanted me to be at home, even though we didnt talk about my nan and we acted normally for the weekend i think my dad just wanted us all there. so although everyones different, i think just being there for your mum will help her, even if you talk about normal things (if she doesnt want to talk about your gran, or talk about your gran if she does) or just sit there with her, she will know youre there for her