Use similar colors of clothing. If one person is wearing white and everyone else is in black you will see the person in white first instead of seeing the group as a whole.
Try to keep in mind that dark colors diminish and make you look thinner. Light colors project and have a tendency to make people look heavier. While white pants may look nice when you see them in real life, in photos they just make hips, thighs, and the lower half of the body look large.
Puffy vests without sleeves also make one look heavier in pictures.
Examples of what not to wear:
Examples of what to wear:
Choose colors that look good on all of you and reflect either the season - if your portrait will be taken outside - or colors that will be complimentary to your home's decor and your family's overall coloring.
Long sleeves are preferable to short sleeves.
White or pastel colors work great with lighter backgrounds.
Casual outfits, in earth tones, are great for outdoor portraits
The less pattern on clothing the better! Try to stay away from stripes, polka dots, plaids, and logos.
Deep or middle tones are ideal for formal portraits.