I often get asked for advice on what to wear for a family photo session. It's a great question. These photo(s) will be hanging on your walls for years to come, so it is an important decision.
For a lot of years, it was common to see a family all in white shirts and khaki pants, or blue jeans. Now that is viewed as trendy and boring. I would not wear white because it makes a person appear heavier while darker colors are slimming. For most people, even thin ones, they prefer to look as slim as possible so I would say avoid wearing white.
What about the idea of everybody matching by wearing the same color shirt, say red or blue, and the same color pants? I advise against it. When people are close together and in the same color it is hard at a quick glance to see where one person ends and the next begins. The optical illusion is to make you look heavier.
What works best is for everybody to be coordinated. Pick a color scheme that you like and that would go well with the wall and room where you are going to hang the picture. For most people, it is neutrals or earth-tones.
Don't wear bold stripes, especially not horizontal
Don't wear the same exact color shirts
Don't wear clothes that are dirty, wrinkled, have holes or don't fit properly.
Don't wear clothes with words, sayings or pictures. - The exception is if you are all on a sports team or being a fan of a team is a big part of your life.
Wear solid colors that compliment each other
Wear clothes that fit you properly
All be on the same level of formality (one person glammed up while the other looking like they are going to do gardening just doesn't look right. You should ALL be casual, or ALL be formal,
Have a coordinated mix of outfits, like one person with a nice button-down shirt and new jeans or khakis with another in a dress.