Hi
You are clearly going to be there for her, which is the the number 1 thing you can do. In terms of what else then from my experience there are a couple of things you can think of.
My wife was diagnosed 3 years ago aged 48. Like your mum she was, and still is, reluctant to read a lot about Myeloma. I did this for her, and was able to pass it on as and when she wanted to know something, it also helped me re-assure her at times.
I also went along to all her appointments with the consultant, as it is hard for 1 person to take in everything they are saying.
Continue to use the forum, there are loads of really helpful people on here.
Finally, remember to look after yourself and make time for you. This will help you to help your mum.
Graham