How to untangle Multiple iOS devices sharing one Apple ID for iMessage
Multiple people in one household can use the same Apple ID in iMessage.  That is how my wife and I have it setup.

On my devices, I have my phone number and email checked for iMessage.

On her devices, she has her phone number and email checked for iMessage.

On the Mac, it is under Messages, Preferences.

On iOS, it is Settings, Messages, Send &amp; Receive.

Which messages are delivered to each device is dependant on which phone numbers and emails you have checked in the preferences listed above.