My whole family has the same Apple ID to prevent confusion. It has worked in the past, however whenever we add a new device we struggle with this problem. And as if that wasn't frustrating enough, messages that my dad sends to my mom show up on my Macbook as me sending the messages to my mom. I also receive my mom's phone calls on my MacBook and my iPhone 6. I have already checked the settings on all said devices and none of them have iCloud message sharing enabled. Additionally, all the devices can be contacted by only one number/email and none of them share anything except the Apple ID. Help would be much appreciated! Thank you!
In Apple world if you are using same Apple ID, your messages and information are shared across devices. That is the reason why you are seeing the messages sent by your family members on your macbook.
I would advise you to create separate Apple ID for every individual or disable iMessage from their personal devices. So that the device that was disabled for iMessage would not sync messages across devices.
Hope this helps!