(Please note: I'm not a expert in Christian/Lewis in particular's theology; this is my impression primarily from the text of the book.)
Peter, Edmund, Lucy and the rest don't know they're dead until the very end of The Last Battle. At the time the discussion of Susan occurs, everyone--including the reader--thinks they simply were pulled through another portal to Narnia, right before, during, or instead of the crash. Peter thinks that Susan didn't come with them because she's no longer a friend of Narnia and didn't come on the train + through the portal with them, not because she's still alive. (The real reason is both, of course.)
"So we got into the train—that's kind of thing people travel in in our
world: a lot of wagons chained together—and the Professor and Aunt
Polly and Lucy came with us. We wanted to keep together as long as we
could. Well there we were in the train. And we were just getting to
the station where the others were to meet us, and I was looking out of
the window to see if I could see them when suddenly there came a most
frightful jerk and a noise: and there we were in Narnia and there was
your Majesty tied up to the tree."
"There's not much to tell," said Peter. "Edmund and I were standing on
the platform and we saw your train coming in. I remember thinking it
was taking the bend far too fast."
"And what happened then?" said Jill.
"Well, it's not very easy to describe, is it, Edmund?" said the High
King.
"Not very," said Edmund. "It wasn't at all like that other time when
we were pulled out of our own world by Magic. There was a frightful
roar and something hit me with a bang, but it didn't hurt. And I felt
not so much scared as—well, excited. Oh—and this is one queer thing.
I'd had a rather sore knee, from a hack at rugger. I noticed it had
suddenly gone. And I felt very light. And then—here we were."
"It was much the same for us in the railway carriage," said the Lord
Digory, wiping the last traces of the fruit from his golden beard.
"Only I think you and I, Polly, chiefly felt that we'd been
unstiffened. You youngsters won't understand. But we stopped feeling
old."
"After the shock and the noise," said Lucy, "we found ourselves here."
So, it's reasonable for no one to say "here's how Susan is reacting to the death of her whole family", given that, well, her family doesn't know they all died. They expect to go back to our world. The last time they saw Susan, they were all alive and Susan was "silly." (Note: Lewis's opinion, not mine, etc.) The omnicient narrator doesn't bring it up at the time because at that point, the reader is also not supposed to know this. The revelation that they're in Heaven doesn't happen until the very, very end of the novel (literally the second to last paragraph), at which point there's enough grand-scale wrapping up to do, and enough on the characters' mind, that the narration doesn't return to Susan.
"You do not yet look so happy as I mean you to be."
Lucy said, "We're so afraid of being sent away, Aslan. And you have
sent us back into our own world so often."
"No fear of that," said Aslan. "Have you not guessed?"
Their hearts leaped and a wild hope rose within them.
"There was a real railway accident," said Aslan softly. "Your father
and mother and all of you are—as you used to call it in the
Shadow-Lands—dead. The term is over: the holidays have begun. The
dream is ended: this is the morning."
And as He spoke He no longer looked to them like a lion; but the
things that began to happen after that were so great and beautiful
that I cannot write them. And for us this is the end of all the
stories, and we can most truly say that they all lived happily ever
after. But for them it was only the beginning of the real story. All
their life in this world and all their adventures in Narnia had only
been the cover and the title page: now at last they were beginning
Chapter One of the Great Story, which no one on earth has read: which
goes on for ever: in which every chapter is better than the one
before.
That's why The Last Battle doesn't bring it up. Re Lewis: presumably, Susan grieved for her family. She may even have turned to religion in her grief, which would bring her back towards the path to Aslan's country, though Lewis said would be a longer journey for her. However, that doesn't change the fact that the reason she's still alive is that she's, erm, a teenager who likes nylons and lipsticks rather than Aslan--that's what makes her "silly" according to Lewis, that state of mind; not her reaction to a specific event.
