the-o-ry [thee-uh-ree, theer-ee] -noun, plural:
a coherent group of general propositions used as principles of explanation for a class of phenomena: The family systems theory is a theory introduced by Dr. Murray Bowen that suggests that individuals cannot be understood in isolation from one another, but rather as a part of their family, as the family is an emotional unit. Families are systems of interconnected and interdependent individuals, none of whom can be understood in isolation from the system.

In a family people often feel distant or disconnected from each other, but this is more feeling than fact. Family members so profoundly affect each others thoughts, feelings, and actions that it often seems as if people are living under the same "emotional skin." People solicit each others attention, approval, and support and react to each others needs, expectations, and distress. The connectedness and reactivity make the functioning of family members interdependent. A change in one person's functioning is predictably followed by reciprocal changes in the functioning of others.

I remember when ever a brother would leave on his mission, or one of my sister got married the house would feel so empty, but it is more then that, the family would feel like it wasn't functioning the same. After a little awhile things, and people would adapt and then it would be normal again. But when that brother would come home or that sister and her husband would come visit us things would be interrupted again.

It is because of this idea of the family systems theory. Everybody has function, a role you could say, and when they would leave their position would be left vacant.