If Heinlein was the gold standard for children's sf in the 1950s, Norton in the 1960s and Christopher in the 1970s, then William Sleator has pretty much dominated the next thirty years. First, he has written sf for children and juveniles (I've refused "YA" for a reason I'll come to in a minute) even when there wasn't a market, and second, he is the only writer working in this area who works with maths, rather than engineering, futuristic adventure or alien contact (actually, the engineering is also in pretty short supply).
In The Last Universe
Susan's brother Gary is dying of leukemia (although the disease is never mentioned). At the bottom of the garden is a maze which you can see from the bedroom windows but cannot get to. The maze is the heart of a family mystery: where are Susan and Gary's relatives? Why is the gardner (Luke) so hostile to any attempts to explore the wilder parts of the garden? And why is the cat called Sro-dee?
When they finally enter the maze Susan and Gary find themselves disappearing into probability clouds and reappearing in worlds where Gary is slightly better. The "adventure" proceeds as the two keep exploring the maze until they miscalculate and emerge into a world in which Gary is far worse and dies, leaving Susan to find the way to the centre (and her relatives) and then, with the help of Sro-dee (Schroedinger's Cat) to escape into another possibility.
In this last universe Gary is miraculously well, and this time dating a pretty girl called Lisa who in a previous possibility had been Susan's nerdy friend. We have moved, in essence, from the world of juveniles in which its the maths that's important to a YA in which the love and the emotion take centre stage. It's an important switch: in that first world Gary demands of Susan that she lock emotion away and focus on the evidence of the garden so that he may find a cure. In this
world it is Susan who is ill and Gary and Lisa refuse her demand for the science and logic of the garden, focusing instead on the YA theme of sorrow and pity and sentiment. In this world not only is it Susan (rather than Gary) who is ill, but stuck in the wrong genre she is doomed to die.