So, in terms of child attachment, a child will try even harder to attach to an abusive parent, it's a survival mechanism.  Also, because of their immature world view, they also tend to think that the abuse is their fault.  So unfortunately, this is "normal" for abused kids and sadly, "normal" for the court system to ignore.

AND, you are too young to deal with this and the fall-out from it.  You are wise to doubt this relationship. 

This is an intolerable situation.  Unclear why a family court would not grant the custody if evidence of this was presented.  Get back to the court and make the accusation.  Ask for a test for the alcohol.  What are the ages of the kids?  They really  need a professional who works with abused children and can help them and also gather details the court will need. These kids need reliable psychological help yesterday with that guilt trip and CYS needs to be involved as soon as possible.  Ask the court to appoint a Gaurdian Ad Litem

The truth is that children will always feel love and devotion to their mothers. In many instances, if they are exposed to abuse, they may even feel their love more strongly for their mother when they are young, hoping that their love can somehow change her behavior towards them. I also think that children can sense when a parent is vulnerable and they move into protection mode. The BM in my life is likely borderline and she has convinced one SS that she is the eternal victim, leading him to constantly accuse DH of maligning her, even though DH says nothing about her. Even when DH tells this SS "I don't hate your mother," this SS won't believe him, insisting "you're not being honest dad, mom tells me all the time how much you hate her." My DH has said to me, "when will I be able to talk to SSs about the truth of the situation" and I've told him that his kids will never believe a word he says because BM has convinced them that he's a terrible monster, even though any interactions they've had with him (and all interactions they've seen between him and BM, where she is always the aggressor) should point to the exact opposite conclusion. 

With regard to how you can cope, if you plan to stay in this relationship, put up impermeable walls between you and her. Your SO should handle everything related to her. 

With regard to how you can support the children, you can try to get them into therapy, but that will be difficult if she and your SO have joint decision-making, as ethical counselors won't see children unless their is proof provided that both parents demonstrate approval for the counseling. Also, borderlines typically like to control the narrative, so counseling is very threatening to them. The BM in my life put both children into counselor because she was convinced they would tell the counselor how terrible my DH was and the counselor would then provide "evidence" that their time should be reduced. When that didn't happen and when going to counseling actually helped on SS have a better relationship with DH, she stopped the counseling. 

You can also make sure that the children know that they have a place to stay with you and are always welcome in your home. They will not likely choose to leave living with their mother, but that is your way to let them know that they have an out. The BM in my cousin's life was abusive and also a heavy drug user, but still had primary custody. My cousin and her DH let both children know that if they needed to move in with them, they would be welcome and also if they needed any help, they could reach out. Neither children took them up on their offer, but doing so allowed them to maintain a good relationship with the children and give them a safe space, even if only EOWE. It's not the best solution, but it's what you can do. 

Kids have been so brainwashed and conditioned to lie and believe there is no abuse, then its real tricky for the authorities to preosecute her.

my husband had suspected his exwife had been abusing their 3 kids as he was sole income earner so left home at 6 am and back home around 9-10pm due to his job. It was the neighbours who could see the exwife treating her kids like slave labour and maids for her.

after the divorce hubby asked his son who was about 13, ss refused to answer and was visibly distraught at being asked that and told hubby never to ask again.

so what else can he do when kids refused to talk about it or admit it. Sd23.5 &14.5 have excused their mums abuse last yr saying it was their dads fault she became that way. That her going psycho bitch during divorce (her choice) was because dad had initiated divorce.

well when your wife has not been a wife or a mum, tortured and abused hubby for 14+ years, sabotaged pregnancies and had abortions behind hubbys back, your kidding yourself to think he’d ever want to stay married to your mum. Oh lets not forget the fact she was cheating on hubby throughout their marriage, but sd’s have made that as ok

Time for DH to give his kids with BM the full meal deal facts on their BM's toxic manipulative bullshit behaviors.  Ever single sordid little detail of fact.  The only way to protect these kids from their toxic BM is for the kids to know it all.  As they continue to progress into adulthood her manipulative crap will only evolve to detract from their lives even more.

We landed on this strategy to counter the manipulative toxic crap dished up by my SS-27's SpermClan.  As he grew up we seasoned him with the facts of their crap in an age appropriate manner.  By the time he was in his mid teens he was doing his own research into their lies and manipulative crap.  He would come home from SpermLand visitation and ask us about something they had said or done that did not pass his smell test.  We would discuss it and he would then go into our Custody/Visitation/Support files to do his own research.  

As he matured and gained confidence he would call their mainpulative bullshit in real time because he increasingly learned the history and facts.  As he called their bullshit, they toned it down.  Now as an adult they know better than to ply their bullshit with him.   He has taken a trip of two to SpermLand to have a heated face to face with the SpermIdiot over his crap with SS's three younger also out of wedlock half sibs by two other baby mamas.  The SpermIdiot has had the ambition to be a gang banger for decades. Since is is inellegiable to be  a Crip or  a Blood since he isn't black, he lives vicariously through his two youngest biracial sons.  When they were arrested for gun violations, gang activity, and drug dealing SS hopped plane, out his SpermIdiot against a wall with a hand at his throat and told him if SS had to return that SpermIdiot would not like the consequences.   This apparently made an impression on the SpermIdiot since according to SS's younger sister (Spawn #2) the SpermIdiot toned down his pushing of the gang banger life style on the two youngest.  

In a trip he made to SpermLand following the forceful wall discussion the SpermIdiot decided that he wanted to take all 4 of his children on a hike in the woods where he expounded on how he wanted them to be good people, respect the planet and proceeded to make an ass of himself as he smoked a ton of weed during the surreal hike through the woods as the SpermIdiot litterally hugged trees in tears. Eventually, SS and his sister just left the other three in the woods, headed back to the trailhead parking lot and drove off leaving the SpermIdiot and the two youngest (then in their late teens and early 20s) to make their way home in the SpermIdiot's two seater car.  They got lost and did not make it home until the middle of the night.  Apparently a stoned SpermIdiot and two stoned spawn have issues with finding their way out of the woods.

The good news is that SS visited with them earlier this year and had a great visit.  All of the kids are in their 20s now and are gaining clarity through having to support themselves. Even the SpermIdiot was apparently pleasant and not overly pathetic during the recent visit.

SS's knowledge of the SpermClan crap has allowed him to minimize the influence of their crap on his life.

Your Skids need the same in order to counter their BM's crap.

IMHO of course.