An issue which has been not adequately explained by the pros is the problem of people's attention when more than one person is trying to photograph a group. If the designated photographer is doing a photo and you have other people standing around with cameras, even if they are just grabbing shots or looking through the viewfinder, some people will be looking where they are supposed to, some people will be looking at their friend with a camera. So in the final product of the designated photographer, different people will be looking different places. The only way around this is to have one photographer at a time working, and it's enough trouble to manage the subject, much less manage photographers and subjects at the same time. Brides or people who rarely/never photograph weddings aren't aware of how distractions mess up group photos and the time efficiency/teamwork required to a too-big list of group photos done.
If your family would like to do it and you've coordinated it with the hired photographer, you should be very fast, and not focus the camera or distract the group while the pro is shooting or managing the group. You could probably bribe the pro to help with your task to ensure everyone's posed properly, but don't offer that until you already have permission from the pro.