Unless you're really lucky this will be the last trophy you need before getting your Platinum trophy. When you start the game you'll have to create your own character and throughout the game (no matter what game mode) you get opportunities to add members to your family and build your very own dynasty. You acquire new Dynasty members through random events or by completing certain secondary missions. You can select these missions by hovering over the blue and yellow exclamation marks that appear above your palace and you select these with . Note that not every secondary mission will give you a new Dynasty member and they're actually rather scarce. Random events will occur through notifications from your advisors that pop up every now and then. If you get a notification about a surprise pregnancy within your family/dynasty, be sure to either organize a wedding or choose "I like a good scandal", cause choosing one of the other options won't get you a new dynasty member (also see Heir and now).
Chances are that you will not have 7 members when you're done with your campaign but you should at the very least be close. If you didn't get your 7th Dynasty member after your trophy clean-up in sandbox mode, all you can do is start a new campaign or keep playing sandbox games until you do have enough members.
Source:
http://www.playstationtrophies.org/game/tropico-5/trophy/110...