Storage sharing in iCloud account
Is it possible to back-up my iPhone to iCloud storage, but not share with other family members that are on my iCloud account?
iPhone XR
If other family members are using your Apple ID and password then they have access to everything that you do. Sharing the same Apple ID is not recommended.
If each family member has their own Apple ID you can use Family Sharing and share selected things. See: Family Sharing - Apple
If you use Family Sharing, iCloud storage is only shared if you opt to to that. See: Share iCloud+ with your family - Apple Support