Rumplestiltskin called it what it is: Sister Wives.  

Unfortunately, I know this well, because that is the situation the BM here wanted, all under the pretense of "what's best for the kids." When we put a stop to the nonsense, I was the one to be forever blamed for destroying "what's best for the kids." <EYEROLL>

The wanna-be sister wife warnings (feel free to add your own; it might save someone): 

BM's and SO's post-divorce enmeshment

Initially, BM came on way too strong with me, acting like we’d be sister wives or something. It was creepy—she had no boundaries. She tried to rope me into keeping secrets from DH, as if my loyalty would be to her, not him. I had recognized for some time that she was about control and manipulation, but had no idea what was coming once we married. 

BM started with parental alienation syndrome within 6 months of our marriage (several years ago). She initially hoped that her punishment would get us "in line" with how she wanted things to be (apparently, she would be Wife #1 and I would take the backseat, while DH would cater to "the family"), but the opposite occurred: we set boundaries and were determined to have a decent life together without toxic interference. It was not easy.

I bring this up because SS recently asked DH why I wasn't on Facebook anymore.

I mentioned here before that I connected with the skids via social media a very long time ago initially out of expectation that we would all get along just fine. When problems (quickly) arose, I stayed connected because we knew we were being lied to (a real dilemma...that wasn't my job, yet the intel was useful although frustrating/maddening).