I posted a little while back about DH, concerns about possible early signs of dementia or ADD, etc. He saw a doctor and was quickly diagnosed with ADD and OCD (the OCD dx surprised me). It was easier for DH to mention ADD to the dr rather than express concerns about dementia, so I don't think that ever came up. It's still a concern. At least he's open to looking at what's going on and doing something about it.  

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.