share storage with family

If you have Family Sharing set up you are already sharing your storage space. What you do not share is access to each other's content. Each family member uses the space on iCloud allocated to the family as part of the monthly subscription, but their content is segregated and private.

At the page I have linked Apple advises: Everyone's personal files and preferences stay private. When you share iCloud+, family members can't see each others’ photos, files, or documents. 

If you wish to share files use Shared Folders with iCloud Drive.