First, you must decide what kind of "look" you're going for. Is it formal or casual? Modern or country? Bold or more simple? This will help to narrow down your (and your family's) wardrobe.
A POP of color is always flattering in a photo because you don't want to take too much attention off of the subjects' faces, but you don't want your pictures to look boring, either!
Here is a sample scenario I came up with for a family of four:
Casual, country, bold!
Color choice- green!
Dad- jeans, striped shirt (with green), brown boots
Mom- neutral-color, casual dress, green pumps or boots, green jewelry
Son- jeans, plaid shirt (with green), brown boots
Daughter- green dress, neutral shoes or boots
Here is a great example of purple/blue hues! Combined beautifully :)
And remember, you don't have to have the exact same color to "match".. I love a great coordinating hue!
Sample scenario #2:
Engagement session
Formal, modern, simple!
Colors- black, red, & aqua!
Man- black dress pants, aqua button-up, red tie
Woman- Red dress, aqua pumps, very little jewelry (you want your ring to stand out!)
These reds look great!
Any form of mix & match will do!
So get creative, but don't forget to look like yourself!