IF you are using the same Apple ID this would mean you are sending iMessage form one account, and they will show on all devices with the same apple I'd. You also share FaceTime and iCloud storage.
if you open settings, messages, turn off your email/Apple ID for messages it will stop the problem.
with iOS 8 Apple has introduced Family Sharing. It allows each member of the family to use their own Apple ID, but still share all purchases made by all members of the family sharing group.