Complex sentence: Two of the family's three children, Elizabeth and Virginia, have yet to be affected by the disorder, while the third child, Ralph, is on the brink of joining the adults, in the isolated cellar of the family home.
Split into simpler sentences: Two of the three children, Elizabeth and Virginia, have not yet been seriously affected by the disorder. However, the third child, Ralph, is on the brink of having to join the mutated adults who have been isolated in the cellar of the family home.