Was that my biggest mistake?

Last night Michael and I went to a Ballroom Dancing class. It was goofy and fun. We learned how to Foxtrot and Waltz. More to the point we learned how we are supposed to do the Foxtrot and the Waltz.
Just as I was getting ready for bed Artie tells me that she has written a letter to her cousin and she wants me to keep a copy of it. She is concerned that her Aunt (yeah, Michael's sister who is the executrix) will somehow use the letter against her and she wants documentation of it. Oh gosh, poor kid. I said OK and asked if I could read it. She said sure.
That was my first mistake.
She says in her letter that the disagreement between their parents (us and her cousin's) has split their friendship apart and she doesn't want that to happen. She added that she has an email address that her parents "can't" look at and she would like her cousin to email her. I was truly put out and I said something.
That was my next mistake.
In all of this conflict that we have had over my mother-in-law's estate my sister-in-law has spued such venom our way. She has convinced family friends how hurtful we are. I stood there and took it from one of my mother-in-laws best friends at my mother-in-laws memorial about how we (every family member execpt my sister-in-law) should back off and leave my sister-in-law alone. I have been working at the Farmer's Market and had family friends come to buy vegetables from where I work and when they realize it's me they clam up and don't make eye contact. I have endured rumors that have been spread about how awful we are to the poor woman over on West Lane and is in charge of her mother's estate.
I have taken it in stride. The family and friends that we are close to know exactly what is going on. They know that every decision that we have made along the way has been a weighty one. How at the beginning we tried to talk to my sister-in-law, tried to get her to come to some kind of balance. We ended up with lawyers because she put us there. She so thoroughly backed us into the proverbial corner.
Now there I was standing in my own living room with a daughter whom I adore and I could have whopped her one good. I was flipping out over the word "can't". My daughter is a terrible speller. I sat down with her and created her email account with her because I type quickly and accurately. I know her username and password. I can look at her email anytime that I want. I do not. I respect her privacy. She purposely chose that word "can't" because her cousin's mother does not hold such respect. In the past Artie has written to her cousin via email and gotten a response from her Aunt. That really irked her. So she wants to let her cousin know, for certain, that she can email her in confidence. I get it, I do.
But...to perpetuate the illusion that we are 'hurtful and bad' people from my own daughter sent me over the top. In all of this the greatest pain for me is that my niece believes that we are awful people. She tells her dad that and she has said it to her mother in public. All of which gets back to me. At my mother-in-law's memorial I walked up to her and put my hand on her back to get her attention. When she turned her head and saw it was me she jumped back and flinched...I could have burst into tears of sorrow.
So, Artie says that she is totally comfortable keeping up this false illusion if it means keeping up her relationship with her cousin. She doesn't feel that she can say to her cousin...'Hey we both believe that our parents are in the right but let's get beyond that and be close again.' She doesn't feel that she can change 'can't' to 'don't'. She says that I am over reacting.
I'm standing in the living room crying my brains out and hyperventilating over the word "can't".
Was that my biggest mistake?