There are a few things I need to note-

1) I'm not entirely sure whether the girl writing the book, or her friend, is the one who is sick. It might be both 2) I remember something about ballet, possibly, or at least a dance class. Whichever girl it is that is sick complains that her legs hurt, which is why the cancer gets discovered in the first place 3) The end might be that they go to a theme park of some kind, but I could be mixing it up with a different book 4) I remember the book being from a certain series or creator, potentially. Something along the lines of Hannah Banana, but I struggled to find something with that. At one point, I wondered whether it might be Hanna Barbera, but that makes no sense, considering their productions, and I've looked through their released books on the wiki to no avail. 5) I don't remember whether the girl who is sick dies or anything, but I would imagine things are possibly a little bleak due to the whole 'This Is Your life' thing. I think that's a full chapter, and she imagines what they would bring up and talks about her family. 6) I read this book as a kid sometime in the very late 90s or early 2000s (probably between 98 and 07), but it could be from earlier than that, I don't know.