I don't really have any advice, except to say I know how you feel. I lost my mum in January, she was 58 and has faced 8 years of treatment. Im sorry he won't get to become a grandad, I luckily had a little boy last year and tour post made me remember to be grateful mum gor to meet him. Life is so bloody unfair sometimes. I know it's easier said than done but try not to focus on all that is to come, just enjoy the time you do have left and worry about the rest of it when you have too. My mum use to tell me to focus on the positives and be grateful for the time we had. God knows how she was so brave. She did leave me little voice notes which I listen to all the time and it really helps. If you feel like you can, maybe you could ask your dad to do something similar. Whatever happens. You will get through it. 

Hello, 

I'm in a very similar situation. Mum diagnosed at the beginning of lockdown with endometrial cancer. She underwent treatment and she was given the all clear. But last week spinal metastasis were discovered, she has a prognosis of 6 months or less. So I totally feel you. 
I live several hundred miles away with a young family. I'm taking each day, not thinking about funerals, clearing houses, never visiting again. It's too much, it gets pushed out of my brain when it pops in. 
I have asked her about her wishes, and been able to sort some stuff out which will help us to help her to die at home. She's taken some comfort from this. I'm being quite stoical around her as I think the worst bit is the prospect of people not coping with her death. 
The next few weeks to months is about quality time, not quantity. Both from a visiting POV but also medical POV. 
Ill make sure I have stuff lined up to meet her needs as they change (equipment, care, advanced directives etc) and that will take the stress out of it for everyone. I'm expecting a decline in her function, condition and I'm going to keep putting things in place to mitigate the loss of function, not try and seek a cure or expect any reversal is symptoms.  But it'll be hard work once we move to the terminal phase and then the aftermath. I'm dreading the aftermath, but again, currently blocking it out. I'll get to experience that in all it's glory in not too long, so it can wait. 
Its utterly ***, but having gone through it with dad when I was 19, mum and I agree that her choosing to be in denial and also accepting every treatment offered isn't necessarily the best way forward. Honest open conversations have been a breath of fresh air. 
Perhaps explain to your family how you feel, and acertain what they might find beneficial or how they are feeling? It's ok if one person tends to do appointments etc. It doesn't diminish how much you mean to someone, it simply reflects geography.  
Also, re. Covid, we've decided that it's not worth getting stressed about. We too were ultra cautious and missed out on time together at the beginning of the pandemic. But now it's about enjoying time together. If we pass Covid to each other so be it, we could easily pass on a cold or flu. Hope that didn't appear to clinical, I'm not the best at writing!  xxxx