This is an obscure suggestion that is probably wrong. But you mentioned that it belongs to a family member which made me wonder -- Is it possible that your "primary" calendar is classified as a "Family" calendar and you also happen to have a YouTube TV account where they share access? YouTube TV automatically invites all members of your "household" to your Google Calendar Family calendar.
https://support.google.com/families/answer/7157782?visit_id=...