I'm so sorry to hear about your father's illness. I can understand your desire to tell your father not to be afraid. I can understand that you and your siblings are fearful, too.
I have recently learned a new way of addressing my fears that I am finding helpful. It is a very simple process of questioning your stressful thoughts and then considering the opposite. It is called The Work (
http://www.thework.com http://www.byronkatie.com).
Once you identify the stressful thought underlying your fear, you could ask these 4 questions about that thought:
Is it true?
Am I absolutely sure that it's true?
How do I react when I believe the thought?
Who would I be without the thought?
Then turn the thought around--consider opposites and examples of how the opposite may be as true or truer than the original statement.
There are free written resources on The Work website, a free hotline to call, etc. On the
ByronKatie.com: The Official Blog for The Work of Byron Katie website, there are videos showing Byron Katie doing The Work with people on stressful thoughts such as "my father shouldn't have died." "I want my cancer to stop growing."
I wish you and your family peace.
Rebecca