Agree 100% with Sandy, BullsOnParade.
You can't be thinking of gear. Either the 24-70 or 50 will be fine, because you've been invited to do this, the working distance to your subjects will be whatever's needed.
Instead, once you've got the setting, background and lighting worked out, obsess with the dynamics of how you run the shoot and what you'll ask the participants to do, because you really have to get the best facial expressions and body language out of them.
If you've never met them before, you'll need to bring in some premeditated strategies if you discover they're all naturally extroverted, or to draw them out to display warmth if they're the reserved type.
If you stop talking to them for five seconds to stare at your LCD screen without explanation, it's a mood killer, so be across all of your skills and settings before the shoot - make it about that family, not your camera and lenses.
Best of luck with it!