Thank you for the sympathy, BlackUnicorn.
Sigma and Pi have a tumultuous relationship. To my knowledge, they sometimes enjoy each other's company but Pi gets very frustrated with Sigma's behavior too at times. Both Sigma and Pi have some lingering issues with each other regarding who gets Rho's time and attention when. Pi lives out of town and wants a lot of attention when she's in town (which is only maybe once or twice a month, aside from special occasions), Sigma is able to see Rho once or twice a week in addition to any friendly gatherings they both end up at. Both are highly active online and get frustrated with Rho if they feel they aren't getting enough time with him chatting online on a daily basis. Putting off one to concentrate on talking to the other is always a powder-keg waiting to blow. Weekend large-group social events where me, Sigma, Pi, and Rho are all present are an easy way to get the drama going.
Sigma isn't calling or texting or emailing me at all anymore, good or bad; if I initiate contact with her for any reason, she will probably resume, most likely with bonus negativity for messing up her apparent coping method of pretending I don't exist. What she is continuing to do that I have a problem with is channeling all relationship drama into constant and repeated texting to my husband and/or mutual friends (often including Pi). It's much easier for her to say awful things in writing than to anyone's face. She has rarely SAID much directly to me that was pointedly abusive aloud and in person, it's that she says things behind my back all of the time, and when she's in touch with me, writes nasty things directly to me some of the time.
I'm not sure who I can turn to at most of these events other than Rho, and sometimes Pi. I'm scared of dragging in mutual friends who are already tired of how Sigma is acting and don't want to have anything to do with it at all, since they're at these events for fun and this drama show is pretty pointedly the opposite of fun. Rho gets upset when Sigma does tons and tons of this stuff in short order, but usually the main result is for Sigma to get hours of his attention as he tries to calm her down, and for Rho to get defensive in turn if I complain about it myself. I could turn to Pi in an acute emotional emergency and she would understand what I was going through but we're not so close that I feel comfortable asking for her aid in dealing with Sigma on a regular basis. :-/