Definition: Given a real-life anecdote of a complex ethical situation, generate a suitable title that describes the main event/root cause of the situation. Imagine that the given text is a social media post, and you have to write the title of the post, so the users can decide to read the main text or not.
Input: I’m on mobile so sorry! 

So first off I’d like to say that my mom throughout my childhood was a great mother. Still is, just I think something has unhinged in her brain the last 5 years. 

About 4 ish years ago, my mom cheated on my dad with his long time best friend. Baffled would not come close to describe how me and my sisters felt. 
Now, I do believe that my parents were great together because they were great parents. They got married because my mom got pregnant with me in their early 20s. They popped out two more over the years and as we all grew older I noticed how different my parents were. One was introverted one was extroverted. 
However, her cheating on my dad (something she has always preached against “I just don’t understand why the person wouldn’t just leave their partner first”) with a guy who is an alcoholic (her own father was an alcoholic) and blaming my dad for pushing him to his arms (he would encourage them hanging out since he doesn’t like bars) has really strained her and my relationship. 
Still love her though. Still talk to her.
BUT my middle sister is getting married in October and does not want that guy “Chad” there. Doesn’t like him. Understandable. But my mom CANNOT accept that we are not ready to accept him. My sister has made it clear that other family gatherings he can attend but just not her wedding. Then my tells us a couple weeks ago that her and “Chad” are getting married in April. So he HAS to be able to come now right? 
My sister still says no.
So what does my mom say to her? 
“If he can’t come, I am not going to come.” 
When my sister told me she said this, I RAGED. I am the oldest and just knowing how she hurt my sister, all of my protective instincts came out and I almost drove 13hrs to yell at her face! I haven’t talked to her yet because only disrespect will come out of my mouth towards her. 
I don’t want her in my life anymore. She crossed the line by just saying that to my sweet sister. 

Oh and since my mom won’t go that means her sister (my aunt) won’t go either. So that’s half our family that my sister won’t have at her wedding over this. 

Am I over reacting? I love my mom but I cannot have a conversation from this point on without disgust dripping off every word. WIBTA for cutting all contact with my mom? 

telling my mom we are no longer on talking terms for giving my sister an ultimatum