Apple one subscribe.Icloud restriction

I quote:

'When you share your iCloud storage, any family members using the free 5GB plan are added to your family plan automatically.

If a family member is already paying for their own iCloud storage plan, they can choose to switch to your plan or keep their own and still remain part of the family. When they switch to the shared family plan, they get a refund for the remainder of their personal plan. They can't keep their current plan and use the shared family plan at the same time.'

https://support.apple.com/HT208147

I don't know whether this is your concern, but family members' iCloud areas are fenced off: they can't access each others data unless the owner has chosen to share specific items.