Hi and so sorry to hear about your situation!
Sigma seems to have some serious cowgirlish tendencies. How does she get along with Pi? How does her husband feel about you? What does Pi think about all this?
Starting to avoid social situations where she will be present is not the solution, it will just give her one over you, like you and everyone whose posted already pointed out. How about having a safe person to go to these gatherings with, who can support you if she starts acting out? Or very simply stating, if the abuse is about to start, that she has no right to say those things to you, and you will need to ask her to leave if she can't behave in a civil manner towards you? If the texts start flooding, NEVER answer! Turn off your phone and delete them without reading.
Me: bi female in my twenties