Unable to add family member to iCloud sharing v2
I have a family plan with the icloud sharing 2TB option. On my Mac and iphone, I can use the iCloud drive and sharing. But when trying to add my mother's iphone (running iOS 11) to the icloud sharing plan, the only option is to purchase another plan, even though she is a member of the family plan.
I have tried removing and re-adding her to the family plan, which did not work.
How do I add her to the existing iCloud family sharing plan?
iPhone 8 Plus, iOS 11.4