Wear solid coloured clothing, so that we see all faces and nobody stands out against each other.
Choose an outfit with muted tones, so that you/your family is the subject and not the clothes. Bright colours tend to make people stand out and look larger. Also, true white and true black tend to draw our eyes to that coloured clothing and away from your beautiful faces.
Choose similar tones for tops and bottoms, all in pastels or all in earthy tones (stick to all dark or all light).
Everyone should choose tones that go well with each-other, so all involved match and stand out from the background.
Wear a top with sleeves at least to the elbow, because arms take up more skin area than a face, and will draw attention. An exception to this is a beautiful, long dress.
Along the same lines, wear long pants or a skirt that goes to the knees. This is also so everybody can sit or bend without being uncomfortable.
Wear dark socks and footwear, because lighter footwear will draw attention away from faces.
Along the same lines, keep jewelry minimalistic if possible.
Do your hair the same way you normally would while wearing these clothes. For example, no fancy up-dos with jeans or ponytails with evening gowns.
If there is going to be a lot of people in your photograph, try to avoid patterns. This is more for extended family photography sessions. If you have a family of five, one person in a floral print can add to your style but try not to have more than one person in a pattern.