parents reading imessages

My step dad has an iPhone 5S, my mother and I both a 4S. We have a shared data plan from At&t. I have a separate apple ID than they do. Is there anyway they can access my iMessages online or on their phone? Is iMessage content stored by Apple or AT&T or any other party?

Also- my mother tracks my location via AT&T Family Maps. Is there anyway I can block her from recieving my location or opt out of it? Thank you in advance