About family sharing.

We tried to find a solution but Apple does not allow that In-App-Purchases can be shared for family accounts at the moment.
Apple Support

What you can do?
Log in with your Apple ID of the other iPhone and try to restore your purchase from StepsApp on the other iPhone. After that, you can change the Apple ID Account. Another way is to write Apple and ask them when they plan to include family sharing for In-App-Purchases.