Sorry to hear about the challenges and health issues that you and your family are facing.
My Father just passed away (Jan 8) without anytime for preparing.
I have also dealt with a close family member being diagnosed with cancer (brain tumor) and became caregiver.
The biggest thing you can do is to grive when needed of the loss or change. Secondly to stay healthy yourself and to try to maintain some sort of normalcy in the face of things not being normal.
Many caregiver sites will repeat this common technique:
Have you been on a plane and heard the flight attendant tell you to put on your oxygen mask first? The immediate response is, “No way, I need to take care of my kids (husband, mother, best friend, stranger in the seat next to me…). The idea clashes with our instinct. What does it really mean? Simply put: If you don’t put your mask on first, you won’t be there for all those other people when they need you. You will be unconscious. The same applies to caregivers. Our natural tendency is to do for others, because we are caring, loving, nurturing, responsible, supportive and competent people. However, just like the oxygen mask, we need to take care of ourselves so we can effectively take care of the people we love..
Celerate their life and make everyday as best as possible for them. Help create the best quality of life as possible for them.
Know it may take a very long time if at all to “come to terms” with it. Try to work through the 5 phases of grief or at least know where you are in it. Certainly get help.