If in doubt, get more in the picture rather than less. If you are shooting with a good camera, you can easily crop out the parts you don't like. However, if you shoot less, you can't really add things back in.
But the more varied you can get, the better.
If your aunt and uncle are standing side by side and uncle Bob is in the back looking drunk as a skunk, might want to zoom in closer to your aunt and uncle to make the picture more flatering. And THEN move in and snap some of Bob drunk.
As its a reunion, trying to snap pictures of multiple generations together is nice, people love to see those. 3+ generations in one shot are great, if you can get 4 or more, well, lucky you!
Keep an eye open for a nice place for posed pictures...good light, low traffic... when people see you shooting, many will start to request portraits "oh, you have to take a picture of me and my fifth cousin I never see Gill!!"