I photographed Bobbi (the matriarch on the top left) at a business shoot recently and mentioned that I also do family portraits. She must have liked her photos, because she then hired me to photograph her whole family. We met in Clinton with the inconic red mill as the background. I gave her some guidance on how to dress and she nailed it. Solid, nuetral colors work best and try to match, I always say. 

You don't always have to match as well as this family did, but you certainly don't want any one person to jump out of the photo. At the very least, try to get everyone to agree on dark colors or light colors. Like jeans and dark tops, khakis and light tops. Whatever you decide, you want it to be balanced. The main thing you want to see in a family portrait is the people, their faces and expressions. Not usually what they're wearing. This family looks great and their portrait is timeless.