I can relate to this, my close grandfather is dying with a tumour behind is eye, lung and last week it spread to the bone. What upsets me worse is that he still has hope just before going for another load of radiation at 77yo. Just spend as much time as you can with him mate, talk about his past, treat him to what little luxuries he enjoys for a treat etc.
Tell your brother to sort himself out and have some respect, he's his father not a payment plan. You all have enough on your mind without having to sort a Will because of your selfish brother. I really couldn't imagine having to sort out your family as well as look after your father, keep it together pal!
I really feel for you mate, just hold in and cherish the time you have left with him.