Jran,
I'm new to the quit, but not to the wife kids and in law situation. If you're situation is anything like mine those in laws of yours love that daughter of yours more than life itself. Grand kids can change people. With that being said, I would not only excuse myself from the "nic" situations, but I would also bring up your quit. Not to go all Jesus on you but the story of your quit and your reasons may just help someone else make the same decision. That girl of yours has powers on everyone that loves her (and for good reason, kids are what we are here for).
So point being: be confident in your quit, not shy. Not apologetic for leaving, but rather inviting them to join. Not worried about the weekend, excited about still being quit on Monday and having more rough roads under your belt to strengthen your quit.
Know that tomorrow s4s is quit with you and will be with my family and in laws (wife's b-day) and each struggle you go through this weekend someone (me) will be following your example.
Set the right one.
May the Quit be with you,
s4s