Once you’ve upgraded to iOS 11 and macOS High Sierra, you’ll be able to share your iCloud storage with every member in your Family Sharing plan (up to 6 members). The sharing takes place seamlessly and you’ll be able to monitor who’s using how much storage.
Families are increasingly using multiple iOS devices to share information, data and media between members. Till now App Store and iTunes purchases were tied to a single account, and your data was tied to a single iCloud account. So if you wanted to sync either of these, you needed to be signed in from the same Apple ID from all accounts.