We can not stress enough how important it is to have all your family present prior to the time you would like to start shooting family photos - even just a few minutes. That way if someone's late (and they will be) it doesn't put everything behind schedule.
It's also good to keep in mind that I don't know every person in your family, so having names and people in order is helpful to ensure no one is unintentionally forgotten.
Finally, a lot of us have slightly more complicated family situations. Please let us know (in the box above) if there's any divorces, or anything else that we should know about. Simply to help me not put my foot in my mouth and say something uncomfortable.