Why do my iPhone messages keep getting sent to my phone and another phone?

Whenever a message gets sent to my iPhone 4, my brother in Vermont also gets them on his iPhone 4S. So like ill be texting my friend and he can see the entire conversation and he can type. I don't get his messages though. I just wanted to know if there's an easy way to fix this because it just started happening about three weeks ago and none of us have gotten a new phone in a year. I'm not sure if it happened when he went back to College and he crossed a state border?

Have youre brother his settings>messages and under send and receive make sure his phone number is selected. For whatever reason if your apple id is signed into his phone and it uses your apple id to send and receive imessages then that should be why messages are going to multiple phones.

Why do I get a copy of my own text messages on my iPhone