My sister (F45) will NOT get over her divorce and her bitterness if dragging down her family, especially her sons (M19, M21).

Throwaway account, just in case. Names changed, all that good stuff.

Long story as short as I can make it: my sister "Kathy" was married for almost twenty years and the majority of that time her husband "Loren" carried on affair after affair. Yes, Kathy knew. Not always the specifics but she knew. The cycle went like this: they didn't talk about it so long as the "family" stayed together, but roughly once every four to five years there would be a BLOWUP followed by a few months of peace as Loren's guilt or shame or whatever took over and he played the part of the perfect family man and my sister played (let's be honest here) the part of the martyr. Their relationship was one of bad kid and scolding mother.

I will give them this, though - they kept their troubles from their boys and were always there for My nephews. All the sports, school events, family trips and vacations - they showered their kiddos with time and attention.

The "cycle" finally broke about five years ago. One of Loren's affairs turned into an actual relationship. Kathy for the first time couldn't guilt him back and BAM, all Hell broke loose. Divorce papers were filed and Kathy came undone - she dragged the kids into it, telling them their father was a sex addict and abuser, she followed his new girlfriend around, she called both their places of work repeatedly, tried to vandalize their cars/homes, and so on.

I sympathize, she's my sister for God's sake and has every right to be hurt and angry. The problem is that after five years she still hasn't let this go. She still makes it a point to rip apart Loren to my nephews and makes them feel guilty when they talk about or see their dad. When the boys see him they'll sometimes come home to her wailing uncontrollably because they "picked him" over her. She still calls Loren randomly and screams at him or tells him the kids have been in an accident or makes up another traumatic event. She goes to Loren's house under the premise of dropping off food she's made for him or something of his she found at the house, then hits the roof when he isn't there or there's a trace of his girlfriend there. One time she "caught" the girlfriend at his place and tried to assault her. Thankfully the girlfriend did not press charges.

The fact is that Loren and this girl are serious, still together, and for whatever crazy reasons their relationship works. No amount of ripping on them is going to change that, all it does is stress out the kids and make the rest of us worry for Kathy's health. She refuses to date and can't stop going on about how men are so awful (she doesn't get why this is inappropriate to go on about in front of her SONS). She seems to think it's healthy for her boys to hear so they can "avoid their father's path."

She's in counseling, but what else can we do to help my sister heal? I don't agree with what her ex did, but Loren is otherwise a good dad to his kids and I think my nephews deserve to have as much of a chance at a healthy relationship with him as they can. I don't want them damaged by something that they didn't have anything to do with.

TLDR: my sister (F45) REFUSES to get over her divorce and still acts crazy toward her ex. It's affecting her sons (M21, M19) and I'm worried that she doesn't seem to be getting better.

Cross posted to r/realtiionship_advice

EDIT TO ADD: wow! Thanks for all the responses guys, I'll try and respond to everyone.