WHAT TO WEAR
For photo sessions of more than one person, I encourage you to coordinate but not to match exactly. Typically, solid colors photograph well. Stay away from clashing colors and patterns, solid whites, and t-shirts with characters/logos/writing. Layered clothing and accessories like hats, headbands, jewelry, scarves, boots and flowers add interest as well (especially on children).
Here you can see a few examples of coordinated family outfits, and something similar would also work for couples or other groups: