Targeting civilians works to demoralize the attacked population, their men who are fighting, and causes refugees which adds to chaos and eats up resources that could be used to resist the invasion.
Er, targeting the families back home does not result in the defending troops doing anything but desiring to kill more enemies. It is self-defeating IMHO. Imagine how you would react if you learned that your entire family had been killed by the enemy if you were on the front lines.