Hi ABoulton7,
Thanks for contacting Apple Support Communities!
We understand that a family member on your Family Sharing plan has a 2 TB iCloud storage plan that you'd like to share with the whole family.
The iCloud storage that is being shared is shared from the family organizer account. This means that if you are the family organizer and someone else in your family group has purchased more iCloud Storage that what you have associated with the Family Sharing group, they can choose to keep their storage or switch to your iCloud Storage as outlined here: Share iCloud+ with your family - Apple Support
"Switch from your personal iCloud+ plan to your family plan
When you share iCloud+, 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+ 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.
On your iPhone, iPad, or iPod touch
In iOS 16 or iPadOS 16 later:
- Go to Settings > Family.
- Tap Subscriptions.
- Tap iCloud+.
- Tap Use Family Storage.
In iOS 15 or iPadOS 15 or earlier:
- Go to Settings > [your name].
- Tap Family Sharing, then tap iCloud+.
- Tap Use Family Storage."
In order to have access to the 2 TB iCloud Storage plan and for the whole family to take advantage of it, the family organizer would need to upgrade their iCloud Storage plan to 2 TB to share with the family.
We hope this helps you.
Cheers!