K.B.
As hard as it is, I would say back away slowly. Your dad may be motivated by guilt or genuine care in not telling the grandfather, but you don't know. It is indeed HIS sister, and HIS father, and he is the one who should be making these choices along with his siblings. Unless you are particularly close with your grandfather, which it doesn't sound like you are, then it's not your place to ask him to be told. If your father is still not talking with you, then it would be challenging to tell him your thoughts on the matter, but you could write an email explaining your argument for telling his father. But beyond that, I don't think there's much you could do.
I think underneath there is some judgment/resentment toward your father, but recognize that even if it is guilt that's driving his current behavior, at least he is being a doting brother. That's fuel for positive change, and maybe it will lead him to make some better efforts in the future. No matter his past mistakes, he is still facing the loss of his sister, and although he's throwing nastiness toward you, muster as much compassion as you can for these circumstances and the people in them. If you can table your resentment for a spell, you may find you will all be better off later.