Bear in mind you will also be able to share emails and Notes and Reminder.
It has the potential to completely mess up so take care. Export your contacts in a computer for safekeeping .
You add your icloud account to her Settings, Calendar. Accounts
Then add her account to your Settings, Calendar, Accounts
You will see you can turn on Calendar but also Contacts, Reminders, Mail, Notes.
You will have to approve the two factor auth code requests when adding the accounts..
You also need to show which accounts are default so that if you add contacts they go on yours not hers.
Alternatively she agrees to only use your contacts etc and turn hers off.