What type of plan do you use? As a calorie counter, I survive family gatherings by keeping my portions tiny, then eating veggies until I'm full -- it gets rid of the "why aren't you trying ______" questions, and lets me enjoy myself without gorging. If I know what the meal is ahead of time, I hop on FitDay and input it, then tweak the portions until I'm happy with the calorie content so that I know how much of each dish I can take. I usually give myself a bit of leeway (I normally eat 1400-1600 per day, but I'll do 1700 one day if there's a special occasion). If you're on other plans, obviously, this sort of approach may not work for you.
Also, do you have any access to running/walking while you're at home? It may not be the gym, and it isn't weightlifting, but it's a good way to keep your cardio up when you're away from the gym. Even just suggesting a family walk after your big meal would do more good than sitting around afterwards -- or go by yourself if it isn't your thing. Even just stretching and situps will be better than nothing!
I recently had a stressful family gathering, so I can totally relate. I hope my tips help, or at least give you inspiration for something that will work for you. Good luck to you. I hope you survive okay.