Apple's Family Sharing feature is not available with In-App Purchases. This is a restriction put in place by Apple and we have no control over it. See this page for more details: https://support.apple.com/en-gb/HT203046
If you downloaded MAVIS from the App Store for free and then purchased our In-App Purchases unfortunately these are not available within Apple's Family Sharing feature.
However, if you are an early adaptor and you paid to download MAVIS from the App Store, MAVIS can be shared with Family Sharing.