iCloud photos security

Uploading photos into the iCloud through the apple photo app I was notified that our family photos were showing up on my in-laws photo library 2000 miles away. Can anyone tell me how this is possible when our libraries are in no way linked? The only common app is friend finder, we have never shared a computer, phone, iPad or laptop. No common family link with apple, completely separate apple ID's and log in info. I have had my apple ID for years, they have only recently moved to apple products. We live in the PacNW they are in the Carolinas. So why now, when we start importing photos from 3-5 years ago do they start showing up on their devices?