Family Sharing - iCloud Storage

Sharing an iCloud Storage is sort of like a shared billing arrangement. Each member's storage is separate and private the same as if they weren't sharing. See the following from Share an iCloud storage plan with your family - Apple Support

When you share a storage plan, your photos and documents stay private and everyone keeps using their own accounts with iCloud—just like when you have your own storage plan. You simply share the space available in the iCloud storage plan with your family members, so you only have one plan to manage.