Both science and religion agree that all humans are related.
Does that mean you want to invite nearly seven billion people to your family reunion? I don't think so!
So who do you want to invite?
It's especially important to know what age groups and genders you're expecting because those things affect the activities and physical facilities you'll need for the family reunion.
Start with age groups. Will there be children? Elderly? If so, you won't be holding the reunion at a remote lake that requires participants to hike twenty miles along rugged trails. Instead you'll want facilities that are easy to get in to and out of. And you'll want plenty of restrooms.
The difference in activities for males and females isn't as big as it is for age groups. But there are womanly interests and masculine ones and you'll want to take them into account. So ask yourself whether your reunion will include only males or only females or some of each.
Write a description of the group of family members who should attend. Then consider the ages and genders that will be involved.