This causes a great deal of confusion and annoyance. We'd happily accept sharing of in-app purchases, or opt-out of Family Sharing altogether to avoid confusion. But we cannot do either due to the app store policies.
You have two options:
- All family members purchase the in-app purchase.
- Request a refund for the in-app purchase, and then purchase the paid versionfor either Apple or Google which does support Family Sharing.