Aww i'm so sorry! You have to remain strong.. I've been through a similar situation before.. I know it's very hard but you have to be patient with your mum and try to help her as much as possible.. You could also try to help your grand mother and take care of her while your mum is away.. this would take some of the stress off your mum because it would mean less calls from your grandma crying.. also try to talk to your grandma and make her feel better..if there are any other family member which you can refer to for help you should also do so.. but the key is to stay in contact with everyone because they'll provide you with support and comfort and vice versa.. and try not to start up any fights or such..
goodluck with everything!
ps. keep listening to Mika, that makes us all feel better and should do the same to you too