Lala494 My deepest sympathy, caring for a child in palliative care, knowing what the outcome will be, is every parent's nightmare, and you are living it
You know you can do nothing for other family while in the situation you are in Is worrying about something you can do nothing about a distraction from all the things you worry about with every reason - like looking after your disabled son?
When the inevitable happens, make sure you allow yourself recovery time, probably a year, before you start taking on other family responsibilities. I am sure your wider family will not want you to be rushing to help them after your loss.
You will need time to recover physically, mentally. You will need to sleep for months, your mind will be like treacle.
In different circumstances my parents lost a daughter and I a sister, and I can remember how we all struggled - and we did not have the months and years of caring preceding the death, as you will have.