Greetings batudc,
Thanks for posting in the Apple Support Communities.
We understand you're looking to access iCloud storage with Family Sharing, but seeing that there's not enough storage available, right?
If we are on the same page as you, let's check what your next backup size is. Since your current 5GB of iCloud storage appears to be full, you could have a bit of information waiting to back up. Take a look at the following steps for assistance with checking this out, and you can find more details here: If you can't back up to iCloud - Apple Support
If an alert says that there's not enough iCloud storage
1. In iOS or iPadOS, go to Settings > [your name].
2. Tap iCloud. At the top, you can check how much iCloud storage you've used.
3. Tap Manage Account Storage or Manage Storage.
4. Tap Backups, then tap the device you're using. Under Last Backup and Backup Size, you can check Next Backup Size. It may take a few minutes for your device to calculate Next Backup Size.
5. If your backup needs more space than the amount you have available in iCloud, you can:
• Delete information that you don't need to reduce the size of your backup
• Upgrade your iCloud+ plan to get more space
6. If you still need help, contact Apple Support.
Let us know if the next backup size is larger than what's available in the iCloud storage plan or not.
Have a great day!