Nobody has to be stuck with having the reunion at their home every year. A nice public area will provide you with a perfect economic solution. Many picnic areas provide large group sites where your family can gather comfortably. Choosing a good picnic area should be done a few months in advance. Assure that the parking at the area of your choice will be adequate.
Many parks offer over-flow parking.
You may want to ensure that all parking will be near your picnic area. Some parks require a per car fee - if your family must arrive in many different vehicles this may not be the choice for you. Some parks will not allow the consumption of alcohol. If anyone will want to drink a beer or two you will need to consider this. You may want to check restroom locations.