This is a new header file, currently only present in the SDK/DDK of Windows 8 Consumer Preview / Visual Studio 11 Beta. I think this header is for splitting the Win32 API into Desktop/MetroApp 'partitions'.
For reference, the winapifamily.h is located at C:\Program Files (x86)\Windows Kits\8.0\Include\shared
C:\Program Files (x86)\Windows Kits\8.0\Include\shared