Start with Mom
I like to recommend starting with mom's outfit first for a few reasons. The first is that I want mom to feel her best and love what she's wearing. The second reason is that it's often easier to find outfits for the rest of the family when you can work off the style and colors from mom's outfit. Be sure that your clothing fits well, is comfortable to move in and reflects your personal style.
Coordinate, don't match
I recommend coordinating your colors or tones rather than matching. (as in everyone wearing a white shirt with khaki pants) Choose one or two colors and mix in neutrals. Did mom already find the perfect outfit? Pull colors from her look and pick tops/dresses/rompers for the rest of the family. In the picture above. Mom has a beautiful blue and white patterned dress. Dad is coordinating in blue and baby in white. Below, you will see big brother in a lighter shade of blue paired with neutral pants and sister in a white textured dress.
Consider a pattern
Don't be afraid of prints. Prints and patterns look great and they can be an easy way to get a color palette for your family's outfits. I do, however, recommend limiting patterns to just one or two members of your family. Fabrics with textures are also a great way to interest your family's outfits.
One print/pattern to avoid- really skinny stripes. These can cause a wavy optical illusion in pictures, called moiré if you're curious.