Currently Being ModeratedJan 2, 2013 10:37 AM (in response to ratcw75)
Go to Settings/Messages/Receive at/ and disable the mail address or phone number for the device that your sister is using.
Also have a look here: iMessage on iPad and iPhone Explained and De-Mystified
Currently Being ModeratedJan 2, 2013 10:38 AM (in response to ratcw75)
Well that's the reason right there. You are both on the same Apple ID. You may want to use seperate ID's because everything that you do on iCloud may be shared to your sisters device. I recommend creating a seperate ID for both of you.