Just a word of caution, make sure that the breast milk is screened. You never know whether the milk is diseased-free, or the mother has HIV, her lifestyle etc. Make sure you do your screening. Even though I have extra, I can't be too sure as I do not want to be responsible for any mishaps.
Hope your niece can get good EBM and get well soon!
Breastfeeding Controversy: Milk Sharing - Parenting.com (Page 1)