The Family subscription is supported by the feature Family Sharing provided by Apple, therefore this Premium model is available for iOS users only.
You must have an iCloud account and be in a Family Sharing group of Apple to share the Family subscription.
Once your iCloud account get access to Habitify Premium via a Family Sharing, you can install Habitify and log into your account on an iOS device and get your Habitify account upgraded.
Then, you can log into your account on any devices even Android or website app while still getting access the Premium features. That's the work-around way we can suggest.