how do i hide a file from the family sharing

There isn't a need to hide files and pictures. You decide which photos, if any, you want to share. See the following from Set up Family Sharing - Apple Support

Share family photos and videos. Open the Photos app, tap the Albums tab, find Family in Shared Albums, and add what you want to share.

If you are asking about sharing an iCloud storage plan, 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.