The last time I was able to get my entire immediate family together was 5 years ago for about a half-hour. The timing eliminated any chance of using a studio photographer. My camera at the time was a K-r, so I had to use a mirror to assess the group's pose and a DIY remote release (
https://www.pentaxforums.com/forums/62-do-yourself/186110-make-wired-release-...ir-camera.html). You wouldn't need to do either of those with the K-3 line as you can use an external monitor - or the Pentax Flu-card with a PC to check the group's pose. And it is easy to cobble up a tiny easily hidden wired remote using the K-3's wired remote port.
The part that is consistent between what I did then and what I would do now with my K-3 is the lighting and focus. You can purchase a 3-light kit relatively inexpensively on ebay with 2 tall light-stands, 1 short light stand, two umbrellas and 3 - 100watt equivalent studio fluorescent bulbs. I only used the two umbrella units as key and fill lights. (I added two hardware-store clamp-on reflectors and another pair of fluorescent bulbs with the same color temperature, to use as rim lights [behind and above the group] for better background separation).
It takes several minutes for fluorescent lights to stabilize, but during that time I could use props (large stuffed animals) to assess lighting quality and make needed tweaks. And once the light output was stable, I used my 30+ year old cheap (even then) light meter in incident light mode in each anticipated face location and made a couple more tweaks.
Because our group (five of us) had to be in two ranks, I knew I would be stopping down. And at the same time I needed a soft focus on the background. So I used manual focus with the stopped down lens to achieve depth of field where I wanted it. Exposure was manual too. Knowing the needed aperture and the values from the incident light meter determined the shutter speed.
Finally during my half-hour time window, the stuffed animals were replaced with our group. With everyone in place, I used the mirror to assess the pose of the other four people, had them hold, moved my head into matching position and used my carefully hidden remote release. The previous post is absolutely correct. If you are focusing on the image in the mirror or monitor during the exposure, you will seem out of place. Since a group of people simply cannot hold a pose for more than a few seconds we repeated this exercise several times and I picked the best of the lot for printing.
While I spent a lot more than a half-hour getting this organized, we achieved our goal of a family portrait within the time we had.