Real Life "The Other Woman"

I apologize in advance, because this is a long one. I'll try to explain as best as I can.

Have you guys seen the movie "The Other Woman", where a married man's wife and his mistresses join forces to punish him for his infidelity? I feel like I'm living a convoluted version of that.

My sister is a beautiful, intelligent girl. She's a registered nurse, and is currently engaged to a colossal douche bag. They have a son together, who just turned one.

When her and the douche bag got together, no one really liked him (he's kind of over bearing and argumentative), but it was fine. None of our business.

They move in together, things were alright, until one day she gets an email and then has a conversation with a girl who claims that her and the douche bag had been involved for three months and that she recently found out that he was living with my sister AND was also in a relationship with a coworker of his. Well, he comes home, she confronts him, and he admits to the coworker, who he says was a year prior and only two or three times, but says the girl she spoke to was a nutjob. She believes him, they break up for like five minutes, and then they're back together. Of course, because she told my family about the affair, we're pretty vocal about his douche baggery. So when they get back together, he's now working extra hard on making sure we're alienated from her.

A few months later, he proposes, she accepts. A few months after that, she's expecting. Fast forward to when she's nine months pregnant and finds out that the coworker wasn't a few times years before, but rather an ongoing affair that had been going on for YEARS. She calls my family, we get over to their condo while he's at work and move her out, including all the baby stuff, and she goes into labour two days later. At the hospital, he's there, saying how this child is going to change everything, he's so sorry, he doesn't work with that girl anymore, he'll cut off all communication and contact, he swears he wants to make their family work. She's in a hard position cause she just had a baby, so she takes him back.

So now, a year later, he's done a pretty good job of alienating her from the family, his son is barely allowed to come over here, at his first birthday party he barely even knew my parents because he's so attached to the douche bag's side of the family, and now for my mom's 60th birthday party this last weekend, they didn't even show up! We live 15 minutes away! So yeah, I'm pissed, and I think to myself, I'm going to contact the other woman, because that's something my sister has never done, because he's always convinced her to "let him handle it". She's a real estate agent, so her phone number is everywhere anyway, so I Googled her and I called her and basically just said hey do you know the douche bag. And she says "Yes of course, he's my boyfriend, why?" Turns out, they've been together for almost SIX YEARS. And he pulled it off because he's a lying psychopath! My sister would work 12 hour night shifts as a nurse. So he'd get home, she'd leave for work, and he would hide everything that indicated that she existed, before his "girlfriend" came over. The girlfriend doesn't think anything is off because she sees him all day at work, then spends the night at "his place" 4 or 5 nights out of the week so obviously he doesn't have a girlfriend, right? And then he told her that his mom was having financial troubles with her house so he was moving in with her, so she started visiting him at his mom's house (without her knowledge I'm sure), but mostly he would go over to her place so they could have some privacy. So basically, the condo stopped existing conveniently around the time my sister went on maternity leave, since now she's home and there's baby stuff everywhere, a little hard to hide.

This girl was FURIOUS and wanted to three way him immediately to confront him, but I convinced her not to, because that wouldn't help my sister, she wouldn't know what happened. And I can't just tell my sister, because he's convinced her that my family will basically say or do anything to break up her relationship (which is fair, but this is true!) So now I have an ally! But a tentative ally, because although she doesn't want anything to do with him, she's scared of making him an "enemy" by blowing up his sick little planet, she just wants to hit the eject button. But she's said if I think of something that lets my sister know but that doesn't make her the bad guy so to speak, she's down to cooperate. So now I need a plan! Please help?

In conclusion, he doesn't know that she knows, but she doesn't want to be the "culprit" in exposing him, but rather have it be a "happy accident".