Why does the primary account holder keep receiving messages from other phones and iphones?

Hello, my father is the primary account holder and he has brought to my attention that he keeps receiving every single imessage and text message that I receive or sent on his phone. He not only receives my messages but both my sisters and mother's as well. My mother does not have an iPhone but my sisters, father, and I do. I wonder if this has something to do with the iMessage or the iOS7 update? I don't want this incovenience to keep happening since my father works early mornings and goes to sleep early. He cannot shut his phone off due to his job and in case of emergencies. My sisters and I on the other hand, are students and we do receive and send messages late at night for whatever the reason. I wonder if Apple can do anything about it, since I already spoke with AT&T and said that it's not coming from their service but rather from Apple itself. This situation also happened to a friend of ours with their primary account holder. Is there anything we can do to prevent this from happening again? Thank you.