A logo is a symbol that represents a business, a family or an event. A logo shows that your family reunion is a real event, not just someone's whim. It provides a way for people to identify who is part of the family if you include it on name tags and t-shirts or other family identity items. It helps people recognize everything that is part of the reunion so it should appear on invitations, envelopes, signs, entrances and everywhere else there is something to do with the reunion.
You don't need a logo for a small reunion that will only last an afternoon although it can make the event more memorable and fun. But you should definitely consider one for a multi-day event or one with a lot of people.
Here are some ways to get a logo for your family reunion.