I would have her screened for lukemia - sustained wbc's over 15000 are pretty characteristic of the disease - don't panic, just be diligent and pursue it until you have an answer, don't let this go unchecked.
Hi
I know a little bit about this topic, that might help you feel better. The higher white blood cell count is probably because your sisters immune system is now attacking the infection that was causing her fever. T cells also get involved in this. I would suspect that the early low count was because her body didn't recognize how to battle the infection yet and is now producing antibodies to get rid of it. Sometimes immune response doesn't happen right away (helper T cells have to recognize foreign particles, go to lymph nodes where B cells are, tell B cells what antibodies to produce, then B cells produce them and they travel to the infection site, it can take a while) and it sounds to me like that is what your sister experienced. I dont know about it being anything diet induced, but then I'm no doctor, just a Microbiology/Immunology student. Good luck to her.
Take care.
Kazzy