If all attempts at reasoning fail then just leave the house for the day and get some spa treatments done. But seriously, organizing for several families at your home on a regular basis is nuts, especially when they don't or won't understand the concept of how much work the hostess has. My parents and relatives in their age group frequently have get-together at their homes, about 15-25 people at a time. They arrange for catering who will bring everything from food to even extra tables, chairs and utensils if needed. People can then enjoy without putting undue pressure on the host. Suggest this for next time.