Hello, I'd like to help you with your question.
It is always tempting to want to hurt the person that hurt you. And it sounds like this woman did
hurt you. She was in and out of your life, she was not honest about her feelings, and she is cheating on her current boyfriend with you.
It appears that she has many issues when it comes to being faithful and sticking with her decisions about her relationships. And she also seems to be putting her feelings first and may not be concerned with who she is hurting by behaving as she does. It is very smart of you to stay away from her by ending the relationship and refusing communication with her. Getting pulled back into the relationship would be very easy so cutting of contact is the best way to deal with it.
Exposing her cheating by sending the emails to her future father in law may seem like a great idea. And it may end her relationship. But it could also backfire onto you. Her fiancé may not believe the emails or he may just defend her anyway, choosing to not believe that about her. She may also try to explain them as you being angry and trying to cause trouble. And even if they do break up, she may just move on to the next guy and do the same thing.
The best "revenge" you can get is to move on and never have contact with her again. You may find that over time, you can put what she did in the past. Use your time and energy to find a new relationship that makes you happy with someone who will be faithful.
I hope this has helped you,
If you're satisfied with my response, please rate me highly. Thanks!