If you do opt for a tree, you have options to make the design distinctive. A summer or beach-themed family reunion could use a palm tree with coconuts representing names. On a more traditional tree, text can be incorporated in various places, not only the leaves but also branches and even underground in the roots.
A different motif for your shirts could be items that come in groups, especially if the item can be related to the family in some fashion. For example a pizza with names in the slices, a box of crayons for a particularly artistic family, or a carton of eggs (with at least one cracked). If music is prominent in your family, names set into various notes could be a theme.
Time is also a popular theme for reunion shirts. A clock face with names at the various numbers is a possibility, but don’t forget other time keeping devices like wristwatches, sundials, even hourglasses.
Your family name itself could be an inspiration. Chef’s hats for the Baker family, a waterwheel with names around it for the Miller’s. An anvil with hammers or horseshoes for the Smith family. Of course if your last name is Johnson, you might want to forgo this option.
While a tree motif can make an excellent design, don’t be afraid to expand your horizons and incorporate other ideas into the mix when coming up with your family reunion t-shirt design.