This isn't really an Android problem, per se.
My wife and I have been effectively sharing our calendars for a few years. I use Android, she uses an iPhone.
The secret sauce is to use Google Calendar.
- We each have our own Google Accounts, and have shared (read-access) with each other.
- We also have created shared calendars for both of our kids, Household events (e.g., when is the trash picked up), Local events we might want to attend, and the schedule for their schools
Getting the calendar(s) into Android is academic (and I just use the standard Calendar app). She simply needed to connect her calendar app. (She is having a problem where sometimes events she creates on her device don't show up, but that's an iPhone issue.)