We have always had the same problem. Family photos make me crazy. Some shots I've seen:

red turtlenecks for all
red for one sex, white for the other
denim for all
denim for boys, red for girls

I have had the best luck with using primary colors for all. One photo we did had the girls in red and yellow and the boys in red and blue. Another time, I found pink and blue for all four kids. Once I really lucked up and found coordinating shirts at Dillards for all four in red and purple. Good luck. Post to let us know how they turn out.