Trying to share photos

With Family Sharing you are sharing your iCloud Storage subscription, but not the content of iCloud. Your iCloud Photos Library will still be an individual Photos Library. To share selected photos with your family add them to the shared family album or share them selected photos as a folder on iCloud.

This will change in autumn when macOS 13 Ventura will be released. Then you will be able to share a Family Photos Library in iCloud. See: macOS Ventura Preview - New Features - Apple

"iCloud Shared Photo Library

Share a photo library with your family

Share a separate iCloud photo library with up to five other people.

Smart setup rules

Share all your past photos, or use setup tools to contribute specific photos based on start date or people.

Smart sharing suggestions

Contribute photos manually or use smart sharing suggestions in a new sidebar item.

Collaborate on the collection

Everyone has equal permissions to add, edit, favorite, caption, and delete.

Relive more complete memories

Enjoy shared photos in Memories, Featured Photos, and the Photos widget.

"

See the article If you can't find your family's shared content - Apple Support

And the section -

Make sure that your device meets the system requirements

To access your family's shared content, you need an iPhone, iPad, or iPod touch using iOS 8 or later, or a Mac using OS X Yosemite or later. If you're using an older device, you won’t see your family’s shared content.

Are the old photos in the family share album, or any other shared album?