I can relate to both sides. My late husband died in 2002 of mesothelioma, a fatal cancer almost always caused from his exposure in his teens to asbestos on a summer job. He lived only 5 months from diagnosis. I am a strong advocate, but inside I was terrified, angry at the cause, I didn't get any sleep, and we had the potential to lose everything. Aspects of his illness nearly killed me. I got help with antidepressants/anxiety meds.
So, five years later, I meet a wonderful man and marry him. Two months later, I get a diagnosis that included the words "most people die from 3 months to 12 months" with no known cure. Well, certainly back to the antidepressants. I also chose to spare myself and my family by asking them to help me conduct life as normal: Lots of love, laughter, get togethers. No sad faces to see each and every day. We can talk about tests and stuff. I just don't want it to be worse than it has to. And I jumped through hoops to get my Will wrapped up so things could change hands immediately.
I feel your pain. Everyone's different and every situation. Certainly one with young children to raise is a big difference and another is economic security. Sad but true, I do hope that she has her will updated and if not, please drum up the courage to ask her to do so for the just in case so the kids will be covered and there won't be additional grief from any hang ups in paying for the mortgage, etc. This is not a time to worry that she will think she's definately going to die. But she knows that she has that risk and she might be shy to bring it up in case that makes you emotional.
I like the idea of writing notes or recording messages. If she is able to record or express to you or write, perhaps she could leave notes or letters for them to be read or heard on certain birthdays and events like graduation, marriage, being a spouse, certain big birthdays -- and for you too. It doesn't have to be prophetic, sometimes the best is showing her recognizable sense of humor. Also, don't be afraid to flirt or show her your sense of humor. They miss that. At times when I thought my late husband was just "out of it," some nurse would come by, touch his shoulder, smile, look happy to see him, and kid him. He lit up like a lightbulb and started talking.
Oh yes, get enough sleep. When caregivers are exhausted, it is too much of a load to bear. Ask for help so you can get teh rest. Even if she doesn't want you out of her sight, this you must do, one way or another. I don't think I could give any more important advice than this. I didn't and it was like having post traumatic syndrome. Don't let that happen.