The only thing I can think for you to check..
On your phone, go to Settings > Messages > Text Message Forwarding.. make sure the switch for her phone is turned off
On her phone, do the same thing, make sure the switch for your phone is off.
This should sort of separate texts sent to the phone number of the specific phone.
But if you iMessage with someone that say, uses an iPad without a phone number.. if they send the iMessage to your shared Apple ID, I believe you would both receive that regardless of any settings workarounds that can be configured.