How do I recover lost photos from iCloud. They are not in recently deleted
Did your sister try logging into iCloud.com to check whether there are some deleted pictures? Use the Apple ID and Passwords to login the website to get it.

If you can't get it from the iCloud.com and the "recently deleted", it's hard to find them back unless you've ever created an iPhone backup. You know the photos that you don't sync to iCloud can be saved to iTunes Backup or iCloud backup. Just restore your device will bring them all back.