Hi Ethan,
I know how annoying it is to be trapped in a family group when the app is gone.
BTW, are you listed as an adult or a child in the family group?
Meanwhile try some quick fixes, hope this works for you.
- If you are an adult: You don't need the app. Go to family.microsoft.com in a web browser, click the three dots (...) next to your name, and select Leave family group.
- If your dad still has his account password: He doesn't need the mobile app! Have him log into family.microsoft.com on any web browser to remove you or manage the group directly.
- If you are a child account & his account is lost: He will need to recover his login at account.live.com/acsr first, as Microsoft requires an organizer to remove child accounts.