“Sitting around with kids you don’t know and the only thing you have in common is that you have a sibling with cancer seems like a group-therapy session,” she said. Since losing Parker to diffuse intrinsic pontine glioma, an especially vitriolic pediatric brain tumor with a zero percent survival rate, Rivers has thrown herself into her schoolwork and dance — a passion she shared with her sister. “I’d rather be as normal as you can.”

“Normal,” of course, is a relative term when your sibling is seriously ill. About 5 to 8 percent of children in the United States will experience the death of a sibling, but the loss is rarely discussed, and siblings of terminally ill children are often overlooked.

There is also little social support for bereaved siblings. A 2010 study in the Journal of Paediatrics & Child Health of 109 major pediatric hospitals in the United States and Canada found that only 48 percent provided sibling support. A 2014 study of young adults who lost siblings to cancer found that most were still grieving two to nine years later.

Beyond the lifestyle changes and the terror of the unknown, the healthy sibling’s role in the family shifts. Parents are in triage mode, and by default, the well child must take a back seat: Their needs simply aren’t as important, or so the thinking has gone.

“I don’t think my parents checked my homework for a year,” said Rebecca Matz, 12, of Mullica Hill, N.J., whose younger sister, Ellie, now 8, was diagnosed with leukemia in December 2014. After telling her that Ellie had cancer, her parents acknowledged that although it wasn’t fair, the family was going to have to accommodate Ellie until she got better.