Hey sognistrada,
I see that you have some questions about using iCloud to share photos with your sister. For the situation you are describing, I would like to suggest that you set up a shared album between you and your sister, instructions on which can be found here:
iCloud: Create a shared album
https://support.apple.com/kb/PH12068
From this album, you will both be able to upload and download the photos you each add. There are some limits on the amount of photos that can be added to a shared album, and information on those limitations can be found in this link:
My Photo Stream and iCloud Photo Sharing limits - Apple Support
https://support.apple.com/en-us/HT202299
If those limits are an issue with the amount of photos you will be wanting to share, the organizer of the album will be able to delete any or all photos in a shared album, but the subscriber will only be able to delete the photos that he or she adds:
iCloud: Delete photos or videos from a shared album
https://support.apple.com/kb/PH12071
Any further information you may need about shared albums can be found in the FAQ in this link:
iCloud Photo Sharing FAQ - Apple Support
https://support.apple.com/en-la/HT202786
Hope this helps, and thanks for coming to the Apple Support Communities!
Regards,
Braden