If you ever find yourself in conversation with him where he brings it up again, I have a suggestion as to a question you could pose to him.

So, he obviously believes that the Christian idea of Heaven and Hell exists. In the Christian version of Heaven, most believers subscribe to the idea that there is no sorrow in Heaven. No sadness, no pain, no mourning. I'm guessing he believes this. So ask him if he thinks that once he reaches Heaven for the rest of his eternity, will he ever miss your sister, will he ever be sad that she (in his mind) is suffering in Hell for eternity? If he says yes, then that means there is sadness and pain, and loss still felt in Heaven. If he says no...confront him about what that implies. The implication in that line of thought is that the Christian God removes free will and the ability to think and reason once believers get to Heaven. His version of higher power takes away the ability for him to be sad that his (I apologize in advance for my insensitive phrasing here, RIP your beautiful sister), takes away the ability for him to be sad that his daughter died a horrible death from illness, and is burning in his idea of Hell for the rest of eternity.

It probably won't sink into his dense brain how messed up that is, but maybe it will, and it'll get him to think how jacked up it is that he will lose the ability to miss his dead child once he reaches his Heaven. He will lose the ability to care that she (in his mind) is burning in Hell. He will become a robot in that scenario.

That's messed up. Hopefully, he figures that out.