Well, from my understanding, the Family pack entitles the owner to 3 -upgrades-, not fresh installs. In order for you to fully honor and abide by the License Agreement, you must first have a previous OS installed on each system, thus allowing an -upgrade-, and not -fresh- install, to occur.
I believe that you can still install Win 7 from the Family Pack though, but it won't allow activation unless you have a previous Windows version install beforehand. The installation itself though shouldn't be any different from a normal install.
Hope this helped!