I believe that only one member of Jeanne's family dies in the book; a sister-in-law not in the immediate family.
The Wakatsukis are a very large family. There are ten Wakatsuki children, so many that, when he is being questioned at Fort Lincoln, Papa has a hard time remembering all their names. Bill is the oldest, then Eleanor, Woody, Frances, Lillian, Ray, Martha, Kiyo, May, and Jeanne. At the time the family was relocated, many of the older children are already married and have children of their own. Bill's wife is named Tomi, and Woody's wife is Chizu; Eleanor's husband is Shig, and Martha's husband is Kaz.
Jeanne Wakatsuki says that three of her sisters bear children while at camp; two of them had hemorrhaged, and one almost dies. This sister survives only because Woody provides blood for a direct transfusion for her, but Jeanne mentions that
"in the case of a sister-in-law of (theirs), who had miscarried and hemorrhaged, no one was able to arrest it in time, and she just bled to death in her hospital bed" (Chapter 14).
The sister-in-law is not named, and presumably is not one of Jeanne's brothers wives, but is a more distant relation. Although the Wakatsuki family is scattered all across the United States by the end of the War, it does not appear that any other of them die.