S.!.
What a GREAT idea... How wonderful!
Do you guys or BIL's have a decent size yard that you could do a cook out in? Each family could be responsible for a part of the meal, as in... you guys will buy the hamburgers and buns, BIL #1 will bring the sides, (beans, Coleslaw, pasta salad, potatoe salad, etc...), and BIL # 2 is responsible for drinks and disposable dishes (plates, napkins and silverware).
Hope you guys have fun and what a wonder DIL you are!!!