How do I delete photos from iphone imported from other iphone?

Me, my mother and my brother share the same icloud. This is because we pay monthly for more storage so that we can back-up all our devices to icloud instead of our mac. Because of this, my photos (from iphone) got imported to my mothers phone and brothers ipad. When I turned off icloud photo library on their devices, the imported photos got deleted🙂

The problem: My mothers photos got imported to my iphone. There is no 'My photo stream' album or anything like that. I have turned off icloud photo library, but it didn't help. Our photos are mixed on my camera roll. I want to delete all her photos off my iphone.

Basically I want all out photos to be backed up to icloud in case any of our devices get broken/stolen, but I don't want them to sync with each other.