Seems fairly straight-forward, based on your story. If he's telling the truth in boasting about his extra-relational sexual exploits, then he's a douche-canoe. If he's lying about it to make himself look good, he's also a douche-canoe, just slightly less so. Either way the guy is a cheater or an egotistical, habitual liar (which wouldn't bode well for your girlfriend's sister's feelings sooner or later).
I think she should know about what was said, otherwise you may feel partly culpable if something bad results of you just "letting things go, man!". I wouldn't want to be friends with someone who feigns ignorance about MY significant other's improprieties just for the sake of my feelings. Sweeping things under the carpet/keeping her in the dark would be to let this asshole win and most likely stomp all over her clueless heart in due time. I prefer the painful truth to a comfortable lie (by omission or otherwise). I also think she would value you looking out for her best interests and even if you got a negative initial reaction, in retrospect would eventually come to appreciate you appreciating honesty and truth and not being so condescending as to think she's not adult enough to handle it, especially if she cares about what you think. And who gives two shits what douche-canoe thinks/does, an extreme defensive or even violent reaction would only further demonstrate you most likely made the right choice.
To the "It's none of your business" folks, the guy made it his business the moment he opened his fat mouth and told him he fucked 4 other girls behind a good friend's back. He didn't ask to hear that, but now that he did, he's been made into an unwilling accessory to douche-canoe's possible immoral acts against someone he cares about and so finds himself in a bit of a moral dilemma.
Like others already said, I'd sit her down and qualify your words and intentions very carefully and just state what was said, not an interpretation what you think he actually meant, as that would be for her to decipher and act upon (assuming she believes them to be in a monogamous "relationship"). At the least, if it only meant her taking a "wait and see" approach and keeping a closer, more skeptical eye on things then it would still be totally worth it in my eyes.
Ultimately you are the most qualified to decide a course of action here, as everyone here offering an opinion is doing so in a contextual/informational vacuum. Go with your own moral compass dude. If it doesn't "feel" right, it most likely isn't.
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