All apps have to be available for Family Sharing. It's part of the Developer Agreement. However, neither Apple or Google allow Family Sharing of in-app purchases.
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 Apple's policies.
You have two options:
All family members purchase the app.
Request a refund for the in-app purchase, and then purchase the paid version for either Apple or Google which does support Family Sharing.