Share iCloud storage with only one family member

Sorry, no. Any family members that are on the free 5GB plan are automatically converted and others have the option. See the following from Share an iCloud storage plan with your family - Apple Support

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.