Family Plan for a Family member already with a subscription
How Do I add a family member to a family plan when they already have their own apple storage plan?
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See the information in Share iCloud+ with your family - Apple Support including the following:
If a family member is already paying for their own iCloud+ 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.