So the situation requires quite a hefty chunk of context, I'll do my best to explain, fair warning I'm not the worlds best writer.

My girlfriend is one of the the nicest people I've met in my life, and honestly it'll regularly be to her own detriment unfortunately (she's by no means a sucker but the people that do these things tend to be good manipulators). This has happened since my girlfriend was a kid her sister has always been pretty selfish and treated most of the people around her as expendable but no one seems to really care and if they do she just cuts them out of her life until she needs something again.

It never really mattered before the last few years since the things that happened when she was young didn't really have any awful results although I'm not a big fan of it. In the more recent years the real problems started.

She suggested told my girlfriend (I'm typing this a lot I'll just abbreviate to MG) that after highschool she could move in with the sister and her boyfriend in order to get out of our home town and into a city area, get a better job and have the opportunity to move out without having to risk moving in with randoms in a share house. Although MG was tentative at first, there was no paperwork or any contracts to sign so it would be easy to move back home if it wasn't working out. Her sister has also told her there will be no set amount she will need to contribute to rent, just that whatever she is able to comfortably contribute would be fine. 

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This turned out to be total bullshit and the entire year the sister was apparently keeping a tab of what she would have to pay (totally unknown to MG), her sister then proceeded to blame her for a multitude of issues that she had previously never really brought up before eg. I didn't even want you here, it's hurting my relationship, you're just a leech etc etc. At this point it really went downhill fast and long story short she essentially kicked her out on new years day and I had to drive 4 hours to go pick her and all her belongs up and take her back home. After this the sister didn't speak to MG for roughly a year.

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After roughly a year the sister starts to talk to MG again but only about superficial things, she would never acknowledge the fact she basically kicked out her younger sister, shafted her financially and made her have to leave her job with little to no notice. MG eventually settled on a superficial relationship saying she didn't really trust her but didn't want to make her parents sad that the two were fighting or make it awkward etc.

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So now its christmas the sister asks for money, it was a small-medium amount for MG at the time, she was working but wasn't made of money. Her father and MG decided to split the money and give it to her (against my suggestion but I didn't hold it against her). Cut to a few months later MG and I moved in together and we were having some money issues and the sister still hadn't payed MG back even after saying she would within a month. We decided to ask her for the money if she was able to, even saying that if she wasn't able it was perfectly fine, to which she said fine and "sent" it but we never received it. This happened several more times her claiming the bank was messing up etc etc (it was obviously bullshit and we weren't pressuring her for it). Eventually we got it but not after some hefty abuse and it really made MG sad that she couldn't rely on her sister.

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Sister has since  broken up the boyfriend she had and moved out (we're not sure exactly when she didn't tell us for ages). She soon found another boyfriend and moved in with him and they got a new place together. I met him he seemed like a really nice down to earth guy. However I was told by MG that he is a serial cheater and has owned up to that fact. About a month after the sister moved in with this new boyfriend MG tells me that her sister has been cheating on him the entire time with her boss who is also married and has a kid, she knows because the sister told her and asked for advice which made her extremely uncomfortable knowing. If the sister was found out she would likely have to leave work, or at least her career would suffer and she would have to move out as well.

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I'm at a point where I could easily tell him because honestly it seems kinda cruel to let it go on even if he has cheated on other girls in the past.  She said she had stopped (after roughly 4 or so months) but she's a serial liar, honestly my main motivation is that I kind of want her to have the stick end of things for once. I debated doing it in a public way that would humiliate her but honestly that's not the kind of person I want to be and also the boyfriend would likely suffer the most.

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Tell me guys would I be the asshole if I told him considering the run on effects as it would likely ruin MG relationship with her sister as she would instantly suspect or just flat out blame MG for it? Even though honestly I want her out of both of our lives anyway and think it'd be good long term.