I was about to suggest the same thing.
Since your cards have been pulled and placed in the VIP office...you can tell your sisterinlaw that you have no choice.
Let them get in line to get checked in...you and your wife go check in at the VIP and then go find the rest of your party.
The Miami terminal is one BIG ROOM...so you won't have any problem finding them.
You have to go up an escalator to get your group picture taken just before you get on the ship....so there will be plenty of time to find your group....