Many fixed-line carriers are providing online storage service, which stores user data through their broadband access network, as a typical application service that utilizes their network's advantages such as wide bandwidth. However, many of these services are not providing unique use cases that are different from over-the-top (OTT) personal online storage services. To facilitate the creation of new telecom online storage services, we propose the concept of a "family cloud", an online storage platform logically connected directly to a household via a fixed broadband access line. The tight coupling of a family cloud and the carrier network facilitates handling of new in-home online storage use cases, including enhancement of existing home telecom services. In this paper, we clarify the family cloud concept and report our extensive investigation of its use case scenarios, our design of a functional architecture that can cover the scenarios, and our application of the architecture to a sample service to verify the concept's validity.