Deleting Local/iCloud Photos

When you are signed into iCloud with your AppleID on several devices, you are always working with the documents and data on all your devices and in iCloud at the same time, independent of the device you your currently using. iCloud is a syncing service, not an independent external storage. Whatever you are doing on any device in an app using iCloud will sync to iCloud anfrom there to all your other devices using the same AppleID for iCloud.

You wrote, that had signed into the iPad with your own AppleID, before you started to delete photos. But you should not have been able to delete her photos from iCloud, if you have been using iCloud Photos with a different AppleID.

Have you already emptied the Recently Deleted album? If not, wait with permanently deleting the photos from Recently Deleted, until your sister has checked her photos.

She should check her iCloud Photos. Then, she should follow, What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support