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Are you both sharing same Apple ID ?
It is possible that she has used your account on her iMessage. That is why she is getting your messages. On her iPhone, go to Settings > Messages. Then tap Send & Receive and delete all your Apple/iCloud IDs from there. If you use shared Apple ID, make sure you don't use it for iMessage.
If she doesn't use iMessage, then turn it off there.
Then on your iPhone, go to Settings > Messages. Then tap Send & Receive and sign in to your Apple ID and password. Make sure you don't use shared ID here.
Then on both phones, go to Settings > General > Reset and tap Reset All Settings. This will reset some settings like WiFi, bluetooth, wallpaper, ringtone etc. But this will not delete any files from the iPhone.
Then again check Settings > Messages > Send & Receive on her iPhone and make sure your account is not there.
Please let me know if that didn't work, so we can continue troubleshooting.
Remember, this is not my final answer. If my solutions didn't work please don't rate me bad or poor. Instead please reply back to me and I will continue to help you until we fix the problem or you are satisfied.
Hi there, I'm just checking. How is it going ?