Hi Mary,
The thing about chemo is that it attacks the good cells and the bad and it can really do a number on the white cells that fight infection. I have had chemo delayed a couple of times due to a low white cell count. And, in late 2006, chemo was cancelled and I was hospitalized with a zero count. That time, it took me more than a week to bounce back.
The shots help alot. I now take Neupogen after each treatment and I have not had any further problems.
The question of how long depends on how long it takes the white cells to bounce back. Your sisters oncologist won't want to further attack her white cells until they have rebounded. It sounds like she is getting good care - the shot, antibiotics and rest are the best things.
One other thing - people undergoing chemotherapy should be especially careful about germs. Seperate towels, lots of handwashing and avoiding crowds and sick people are a really good idea.
Good luck to you both-
Laurie