Always be yourself. You should wear something that you are comfortable in. Wear something that truly represents who you are. If you are comfortable with your outfit, your happiness & comfort will show through in photos, which is super important.
Complement each other in colors & style. Unless you & your family members purposely dress the same every day, I wouldn't recommend dressing the same for your photo shoot. Pick at least three colors that go well together & go from there.
When choosing patterns, it's best to balance out the patterns by also wearing solid colors. If you are wearing prints then the other person should wear something solid. Limiting the patterns to one parts of your body is best.
Make sure you & your family members are dressed as if you're going somewhere together. One person shouldn't be completely dressed up while the other is dressed down. One person shouldn't be in their Sunday best while the other is wearing clothes they usually wear to clean the house. If you wouldn't wear it out to eat, on a date, or to church, then it's not the right outfit.
Think about your location. If you're shooting around a bunch of hills & cactus, make sure you are wearing colors that pop & goes against the browns & greens of nature! And always wear shoes that are comfortable & will protect you from any stickers. If you're shooting against old buildings, wear colors that are bold in colors in contrast to whatever color the buildings may be.
Do not wear clothes that have logos & characters. This dates your photos, which will not go over well. You want your photos to be as timeless as possible.
Do not wear neon colors. The colors will reflect onto skin, hair, other pieces of clothing, which is not flattering.
If you need help figuring out what colors go best together, you can check out my pinterest board. On the pinterest board, there is a color wheel chart that may be helpful for you.