Alex’s Lemonade Stand: Mailers for siblings and a bunch of resources:
https://www.alexslemonade.org/childhood-cancer/for-families/supporting-siblings
Camps for cancer families (including some for siblings!) :
https://www.alexslemonade.org/childhood-cancer/for-families/summer-camps
ACCO sibling resources:
American Cancer Society: https://www.cancer.org/cancer/childhood-cancer/helping-siblings.html