iCloud sharing

You have signed in with same Apple ID and password on the same network on both Mac on the same network , the calendar is synced with iCloud https://support.apple.com/en-in/guide/icloud/mmb1de40d9/icloud

in your Mac address book her contacts details are saved including the mail address , trusted number ...

So when ever you share a a calendar ( just right click on it the calendar and share with the person it , the person has to accept the invitation that is received in mail ( IMAP account or if accessing web account ) , the calendar sharing starts .

To prevent calendar sharing see the article https://support.apple.com/en-in/guide/calendar/icl32362/11.0/mac/11.0

https://support.apple.com/en-in/guide/icloud/mm6b1a8f9f/icloud

If you are using the same Apple ID then any service using that Apple ID will think it is just one person with multiple devices. Some services let you sign in under different Apple IDs but others use the main one set on the phone. Ultimately you should think about each using your own Apple ID and sharing items with Family Sharing.

Create an Apple ID - https://support.apple.com/HT204316

Set up Family Sharing - https://support.apple.com/HT201088