iCloud Photo Library is a syncing service. Any photo you delete from a device where iCloud Photo Library is turned on will also be deleted from iCloud and from any other device connected to that iCloud Photo Library. You can potentially reduce the storage space required on a device by turning on Optimize Storage. See: iCloud Photo Library - Apple Support
Apple does not currently offer cloud-only storage for photos. If that is what you want or need, you may want to explore 3rd party options such as Google Photos or Dropbox.
Regarding her photos that are on your phone and your iCloud Photo Library, you will need to transfer them to her. One way to do that is via AirDrop: Use AirDrop on your iPhone, iPad, or iPod touch - Apple Support
Once they are on her phone you can delete them from yours and they will also delete from your iCloud. Once you have her photos on her phone and only your photos on your phone you can then share the photos that you want to have on both phones. See the following from Family Sharing - Apple Support
When you set up your family, a shared album is created automatically in the Photos app on all family members’ devices. Everyone can add photos, videos, and comments to the album whenever they like and get notified when something new is added.