Clothes – For siblings, the children don’t have to match perfectly, just coordinate. You can’t go wrong with dark, rich monotones, which drive the attention in photos to sweet faces and darling expressions instead of loud prints or colors. If nothing else, pick a color that compliments Eye colors. Try and avoid shirts with logos or patches.
Kids- Clean, clean, clean. The cleaner the kid, the better their photos will turn out! Everything needn’t match, simply coordinate with your guys' outfits. If the Dad is in a T-shirt, don’t put the kids in dress shirts – make it make sense. Just remember basic stuff like clean nails and faces, wipe away eye boogers, wipe noses.
Guys- Have a fresh shave or trim, (unless you normally sports a beard.) Make sure nails are clean and trimmed. Pay attention to eye boogers and sleep crusties. For wardrobe, you don’t have to perfectly match the rest of the family, just wear something that coordinates.
Girls – Women know what to wear in general. If you wear jewelry, just be aware of it twisting or turning.
This is just some basic stuff that most people know and will do but I like to throw it in there just for insurance.
One person wearing striped patterns, next to the other with flannel patterns...will clash badly. If one of you wears a print, the other should wear a solid.
Fluorescent clothing usually casts a color on the person next to you. Wether on their hair, neck, or face. Best to avoid these.
If you want to go with colorful outfits, check out a color wheel. Match a primary color with a complimentary color (one on the opposite side) and problem solved. ;D