Unfortunately it's not possible to share the subscription to Instories via family access.
Via family access, you can share such things as:
the application itself - this is included by default, so you can share the Instories app (but not a subscription!) with your family members.
the subscription inside this application (not available in our application).
Your subscription is connected to your Apple ID. To use your subscription on another iOS device, you need to log into this Apple ID in the device settings. Then open our app, tap on the settings menu in the upper left corner, then "Restore Purchase".