I would always put aside approx. an hour for a family shoot. However this would vary depending on the size of the family and the age of the children. Young children can get tired of the camera very quickly in these situations. I will work very fast to get the best out of them.
The most important thing is that you are comfortable with what you are wearing. I like to keep my photography timeless so I would recommend wearing simple neutral colours. Smart casual works best. Please try to avoid heavily patterned items of clothing or anything woth logos, this also applies for siblings.
Wherever you want! Have you a special beach, park, garden, farm.