What is attracting them to the deck, do you suppose? Do your in-laws have bird feeders or something else that might be tempting them? This may not be the best solution, but I had some friends that actually fed the racoons kibble from a feeder, far, far away from their patio. That seemed to be a good compromize for them.
Buster - Rescued male - fawn, docked, cropped, 11/1/99 - 6/7/11
Shaylea - Rescured female - brindle, docked, floppy, 12/1/02
Tyson - Rescured male - white, docked, floppy, 3/19/04 - 4/23/13