Hmm. I was just thinking about this. I would say that families are not teams, because they are not "put together for the express purpose of accomplishing a task." They can sometimes act as a team, however. When my father died, my dh and I, sister and BIL, and youngest sister worked as a team to take care of the many tasks that must be done after a death and make sure that my mother had all the help she needed and was involved in every task and decision that was important to her.
Now, my married sister and BIL are very different from the rest of the family and there have often been tensions. But during that time, everyone put those things aside to take care of business. We planned, delegated, and reported back to the others with no problems. We were grieving, but we had a mission to accomplish.
OTOH, no one was *the* leader. That role changed according to what task was being handled. I don't think that teams which are put together in a work-related setting can operate that way.