Anayia Skipper, who is 20 months old, and her 5-month-old brother, Xander, are believed to have a disease called HLH (hemophagocytic lymphohistiocytosis).
The majority of HLH cases in children under a year old are genetic.
A bone marrow transplant may be the only chance both children have to survive.
Typically, siblings are a perfect match for a donor, but in this case Anaiya and Zander can't help each other.
A donor drive for the children is scheduled for Tuesday from 9 a.m. to 6 p.m. at the University of Southern California.
