Be a Poser
Photographing someone directly from the front is often the most unflattering angle: The subject looks heavy, and his or her facial features are squished into two dimensions. That's why few people look good in passport photos.
Angle is a subtle trick to control how good someone will look in your photos. You know how some people are known for being photogenic? Often, it's just because they know how to pose.
When setting up a family portrait, take the time to direct the members of your family into poses ("move a bit to the left," "cock your head to the side," and so on). If your uncle is holding his arms straight, ask him to bend them. If your father is standing rigidly upright, have him shift his weight onto his right leg. If the picture is a close-up, have the subjects lean slightly toward the camera. Instead of your sister keeping her hands clasped, have them resting on her hips, a prop, or the shoulder of someone next to her.
Also, think about what your family is going to be wearing. Dressing your entire extended family in matching shirts is a little much, but you should plan some sort of color scheme that won't clash. You don't want a situation where everybody is dressed in dark browns and your sister suddenly shows up in a red dress. Also, avoid short sleeves and short pants. The eye is naturally drawn toward bare skin, and it can distract attention away from a subject's face.
Posing is a great tool, but don't overdo it. It's okay to be meticulous about posing your family for portraits, but if you're constantly forcing your family to strike a pose ("Do that one more time for the camera, honey"), you're just going to aggravate them -- and aggravated families make for poor photos.