How did you manage to establish boundaries with *very* HCBM? (long post)

There is SO much context within this, but to sum it all up, and I’m paraphrasing the situation A LOT here.. we are dealing with a very unstable BM who is terrorizing our lives. SO (35m) has 2 young children with her (5m and almost 2f) who came about under very deceptive circumstances. Again, long story, but I can elaborate in comments. We also have an ours (~8 months) and another on the way early next year.

BM has had a VERY unhealthy obsession with my SO since they met. He was in his “bachelor stage” in life and she lied about falling pregnant when he wanted to cut things off with her. How she eventually ended up pregnant anyway falls under the above deception I mentioned. Anyway, not knowing it was a lie he tried to “do the right thing” by helping provide and giving her a place to stay solely because of the child, because if not for him she would be on the streets and would have no problem dragging the kid(s) along with her. For the past almost 6 years she has refused to work. So, he was very much stuck in that situation. Still is, financially he has to help her so much that I take on 80% of the expenses in our home.

When he eventually met me and told her about me, she didn’t take his relationship with me well at all, even though they had not ever been together. Ever since, it’s been her life mission to cause us misery and turmoil. She will show up any day and any time with the kids, pounding on the door yelling and screaming, sometimes at unholy hours- I mean 2am, 3am, late night, whatever. She will blow up his phone any and all hours of the day on a daily basis causing drama over any little thing. Sometimes it’s very psychotic, going off about how he “left his kids” even though we have them more than half the week. The kids live in filth, there is garbage in bags in the house, on the porch, maggots in the sink, rotten food in the fridge.. stuff like that. The kids NEVER sleep, they are hardly supervised so it is hell on earth when they’re over at our house. They are messy, up all night, scream and cry and whine all day long, sleep very little and very behaviorally troubled. Our house is constantly trashed the days they come over.

I have tried everything to get him to set some boundaries- at the extreme go to court, at the least stick to the days we get them and cut off contact with her during those days unless there is an emergency so we don’t have to deal with all the drama. I haven’t had much luck with that, because if he tries to minimize contact she will just show up at our house, screaming and causing a scene, and none of it makes any sense. She just comes over with claims he’s abandoning his kids for another woman (we have them more than she does) that he’s a terrible person because the kids cry for him all the time (probably because their home life with her is nothing but turmoil) saying horrible things about me, purposely losing jobs or saying she has no electricity, car trouble, literally anything to have his constant attention, guilt and support.

If he fails to give her the attention she seeks, she uses the children as weapons. Threatens to quit whatever job she has at the time, keeps the kids away, threatens to call the police, all kinds of crazy stuff.

When our baby was born, she dropped the kids off the moment we left the hospital and never came back for them my entire maternity leave. She purposely wanted to make sure he had no time to bond with the baby. They also had Covid which she knew, so our newborn baby and I were sick with Covid for an entire month, which killed my breast milk supply. She tells the kids that daddy has a new baby and a girlfriend and you guys don’t mean anything to him, so now when they are here they are SOO clingy, he can’t spend any time with me or the baby because the kids will scream and cry and jump all over him any moment they don’t have his undivided attention. We basically have to live separate lives in separate rooms of the house because of this.

I feel bad for these kids, it’s all very unhealthy for them, but my goodness.. the stress and frustration this all brings has me at my wits end. We can’t even try to give them stability on the days we have them, because even on those days it is ALL about her and constant damage control. Anything I say now he takes very offensively, like my trying to set boundaries is going to cause him to lose his kids and make things worse for all of us. if he goes to court, she will full on give up on them and there is no way we can have them full time. That much is true, we can’t afford day care for multiple children and we both HAVE to work.

I really don’t know what to do here. I guess I just have to deal with this and hope she eventually gets tired and goes away? That doesn’t seem likely though, and the imbalance it creates in our life is unbearable. I feel like I am doing it alone, while he is still very much stuck in the past only able to focus on her and their kids. I sympathize for him, but I would do anything for it all to stop. Any advice here?