Some of the best family photos are captured outside of a session with a professional photographer. Holidays and family gatherings are perfect opportunities to grab some true-to-life candids, but that doesn't mean you can't plan your looks to create a cohesive image. Independence Day festivities in particular lend themselves to coordinated style because everyone is sporting their patriotic flare.
To avoid an image that was obviously taken on the Fourth of July, I suggest limiting any obvious flag patterns and using varying shades of red and blue. This also makes it more likely that you can pull pieces from your family members' existing wardrobes. This is a plus for many reasons - you won't spend money on items they may never wear again, you know they wear and like the clothes you select, and you can do it on the sly so they're not grumbling about doing a family photo before you even leave the house. ; ) No one will be the wise when you casually suggest that Uncle Bill (the family shutterbug) take a photo of your crew.
All of these pieces were pulled from J. Crew Factory, but they're all fairly basic summer items that you would pull from your family members' closets and encourage them to wear to a nice backyard BBQ.
Everyone has a little pattern and/or texture in his or her look with various shades of red, blue and neutrals. When mixing patterns, a good general rule is to mix the scale so that you don't have multiple bold, all-over prints and to imagine how the prints will look alongside one another as family members pose. In this example, if the husband and wife posed together, the scale of her printed dress will not compete with the bolder stripe on his tee.
My final tip for an impromptu Independence Day family photo is to request the shot as soon as you arrive at your destination - before the ketchup, mustard, and popsicles add extra flare to your ensembles.