The photography is the easy part. Getting everyone in the same room on the same day is the job.
Pick the date around the hardest person to move
Usually an elderly relative, or a family member travelling in. Build the date around them and everyone else will adapt. Weekday mornings are often easier than weekends and the studio is quieter.
Write the list
Every grouping you want, with names against each. The full group, each nuclear family, the grandparents alone, grandparents with all grandchildren, siblings, each couple. Twenty minutes of thinking beforehand and the session runs itself.
Without a list, the session runs on whoever happens to be standing nearby, and you discover the gaps three weeks later when the gallery arrives.
Clothing across fifteen people
Pick three colours and send them to everyone. Not outfits — colours. People wear what they own and feel comfortable in, within that range, and the group holds together without looking staged.
One person in a bold colour will pull the eye in every frame. If that is intentional, fine; if it is not, mention the palette early.
Pacing an hour with a toddler and an eighty-year-old
Full group first, immediately, while the children are fresh and everyone is still tidy. Then the older relatives, so they can sit down or leave. Then the couples and the children, which can take as long as they take.
Seating is brought into frame rather than hidden — a grandparent seated with the family around them is a better composition than everyone standing in a row anyway.
What tends to go wrong
Sessions booked too late in the day, after a lunch, with small children who needed a nap an hour ago. Mid-morning is almost always better.