I'm on iOS12, my wife is on iOS 13, she's in my family group. Today, I decided to share photos for the first time. On my iPhone 6, I went to the photos app and created a new album in 'shared albums'. Then I added 128 pictures to it. I then got a window with a pie chart saying "Vorbereiten..." ("preparing"). It has taken a very long time to complete this process, so I assume the images are physically downloaded and re-uploaded.
Why doesn't icloud just give permission to the photos I share in my family group?