As sherrard just said, she will read this letter and, basically, come to the conclusion that she has won... which she clearly has!
You say you are happy, but you can't be. I understand you are angry, but you need to deal with this in some other way. Her poor parents have done nothing to you, and they do not deserve this. Why would you even involve them?!
Honestly, like pretty much everyone else has said, leave it be. Get on with your life with your family and stop living in the past.
No good will come of this and, as Joko has mentioned, this really could open up a can of worms.
Do you really think she will let you get away with involving her parents in all this? No chance!