Unintended Information sharing across devices

I just upgraded to OS Catalina 10.15.3. While trying to copy a quote from a pdf into a Word document on my MacBook, I pasted text I didn't recognize. It turns out the text was something my wife had copied while on a different computer in the other room only seconds before. She is working under a different account profile, though my account is the master account for the family.

How do I find out what happened and manage what gets shared across family accounts and what doesn't? I tried family sharing under system preferences, but that appears to be only about app purchases, locations, iCloud storage, etc. - all stuff I want to share.