[Spoiler] Kat theory

Given how religious her parents are, coupled with her not showing the usual signs you see of someone on chemotherapy like her losing her hair and being fatigued, it would not surprise me if her parents were choosing to treat her with prayer and other faith based methods instead of regular medicine. That would also explain why she hates her parents so much: by denying her treatment, they are dooming her to an early death that could theoretically have been avoided, since in 1995 the survival rate for leukemia was roughly 85% with treatment.

EDIT: I have been made aware that the 85% I cited is likely inaccurate. I admit that I simply googled what the survival rate was in 1995 and used the first result without looking deeper into the numbers.