Really sorry to hear you are going through this. 
We dealt with this at a rather closer range as dd made a couple of suicide attempts around the age of 14/15. Obviously impossible to conceal from her (3 years younger) brother, especially as he was the one who found her the first time. But in a strange way perhaps that has made it easier as we have been able to work through it as a family. At times it has affected him badly, but he has also been able to call on the support not only of dh and me, but of school and of his best friend. He did have flashbacks for a while, and eventually even spoke to dd about it (they are very close).
I would tell your dc what is going on but definitely avoid any speculation about the future. You really do not need to prepare them for something that might never happen and which doesn't have any kind of timescale. Preparation is for when something is imminent. But talk to them about what is going on now, how it is affecting their db and you, what you are doing and if there is anything they can do; let them feel that you are going through this as a family.
My ds is also sitting his GCSE's now. His fears about his sister have lessened in recent years, but his grandmother, whom he loves deeply, is dying and in a great deal of pain. He knows what is going to happen, he knows what she is going through now, he knows what he can do to help (not a lot as she does not want visits, but he has written to her). Being part of it makes it more manageable.
Otoh when grandma first developed cancer we did not prepare him for her eventual death- because there was no knowing when that was going to come and in the event it took years- but talked about how it was impacting her life then and about what was being done.