Cancer does suck, and I offer you well wishes in getting through the next few months.
Well, I come from a different point of view. I am a young cancer survivor - diagnosed after my daughter's birth 2 years ago. I was lucky to have a good prognosis and have successfully completed treatment, but I know, too well, that could change any day.
My best advice is to contact an organization like Imerman Angels (imermanangels.org). Becoming a survivor was really hard for me. They're a non-profit that supports people and loved-ones dealing with all stages of cancer. They'll connect you with another daughter who's been in your shoes and help you go through the process of dealing with her terminal diagnosis. For me, I was matched with another 30-something young mother diagnosed with stage II Hodgkins. It was really helpful.
As far as what to do: ask her? People think cancer patients are afraid to talk about their condition. In reality, we probably spend more time making other people feel good about it than we receive. I think my candor took some people by surprise.
But, I'd ask her what she'd like to have in remembrance. If she wants videos of her reading her favorite children's stories for grandchildren/greatgrandchildren to view down the road. It's OK to ask because it will be a memory for you to have down the road and what better way to commemorate her than as she wishes? Ask if there's a favorite photo, have her write a story about a favorite memory, etc. But, let her be part of the process for as long as she can.
She knows this is going to be harder on you than it is on her. She is likely doing as much to comfort you as possible because of that.
Best wishes to your family. There are many, many organizations dedicated to helping you all through the process (like Imerman Angels).