I am trying to remember a story where the older girl character is sick with something that needs a operation to save her life and she keeps it from her friend who is a boy she refuses to date due to her illness she eventually tells him and he goes to the hospital to see her but gets there to late. I thought it was Aroslav's LNDtH. But it's not so I'm hoping someone here remembers better then me.
https://storiesonline.net/s/42289/rewind
By Don Lockwood has some similarities.
Beffy / Beth Trovino, adopted big sister to the MC, is dieing of cancer. She tries to hide from the MC who is emotionally fragile. He does see her in hospital before she dies.
It is an interesting theme of do overs that there are limits to what you can fix.
PT Brainum's John the Genius explores this.
https://storiesonline.net/series/1434/omniscience