General tips:
All family pictures look better with coordinated clothing.
Although matching outfits do look nice, they are not a necessity. Clothing themes or coordinated colors work just as well.
Additionally, solid colors almost always look better than patterned or "busy" clothing.
Seasonal tips:
Winter
Light colors such as cream, light pink, light brown, and even grey create that "dreamy" winter wonderland look.
If possible, try not to wear gloves or mittens unless they work with your chosen color scheme.
Spring
Spring is a very colorful time of year, your photoshoot wardrobe should reflect that.
Stark, bold colors such as deep blues, bright pinks, mustard yellows, or purples look stunning against bright and flowered backgrounds.
Summer
Summer is often hot and stuffy, which is why clothes that are light and airy look and feel splendid.
Some popular options are soft greys, creams, whites, soft blues, and soft pinks. Navy blues, denims and darker greys also work well.
Fall
In fall shoots, you want your family's wardrobe to compliment the gorgeous colors that are all around.
Some fabulous color choices include greys, maroons, mustard yellows, browns, rusty oranges, forest greens, eggplant purples, or even deep blues.