Why do i keep getting other people's iMessages?

My sister got a new iPhone 4S and a screen popped up on my iPhone 4.It said something about getting imessage with 614-567-3879 (my sister's phone number) and so i though that meant whether i wanted to have iMessage when talking to her, but i didn't read all the way through. So i hit yes, and now whenever she sends a text to someone or somebody sends a text to her, i receive it to. How do i turn this off?

You must have the same Apple ID. One of you needs a new account.