I alway’s recommend photographing mom + dad separate from the family. After all they are the reason the family exist. Quite often it’s been years since their last professional portrait and it only takes a few minutes. I know their children and grandchildren will appreciate the portrait down the road.
Clothing play’s a large part in family portrait. You have to think of what you’re doing with this portrait. Which wall will it hang on? What are the colour’s of the room’s decor? Is it going on shelf? Framed or floated on an easel? As someone with an interior decorating certificate I can guide you for the best location of your portrait, and what colours you should be wearing to compliment the space. Of course these are only recommendations, as you can wear what you like.