Hi Dana and welcome to the Disney Parks Moms Panel.
What I have learnt from my many visits to the
Walt Disney World Resort is that Cast Members will always do their very best to accommodate any requests made by Guests, wherever possible.
In this instance, I would recommend that your entire travel party (your family and your in-laws) arrive at the restaurant in question at around 7:45 PM on the day of your Advance Dining Reservations. Upon arrival, explain your situation to the Cast Member at the check-in podium and ask if there is any possibility that you would be able to be seated together.
Although nothing can be guaranteed, I have a feeling that they will be able to arrange something for you, even if it means that you have to wait a little while.
Wishing you a magical trip filled with memories which will last a lifetime!
Emma