Why am I still being billed for iCloud services for another family member on my plan when I am paying for the family Apple One plan?
Why am I still being billed for another family members iCloud storage when they are on the Apple One family plan?
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The family member is not required to share the storage in your Apple One plan. They can choose whether they want to or not, See the following from: What happens to your iCloud storage plan when you sign up for Apple One - Apple Support
If you belong to a Family Sharing group and a family member subscribes to an Apple One Family or Premier plan, you can choose to keep your current iCloud storage plan or use the shared iCloud storage in Apple One.