According to an iMore article, Apple is set to allow developers to add in-app purchases and subscription to Family Sharing. The article also stated, “Once this change comes into effect.....”
With that being said, I don’t know if the ability has been granted to developers or not. In addition, the article stated, “The change wasn’t mention in the iOS 14 or iPadOS 14 announcement....”
NOTE: The article is dated June 22, 2020 so again, I don’t know if the permission to developers have been granted or not. Hopefully, someone who knows will reply soon.
Source:
https://www.imore.com/apple-adding-a...family-sharing