I highly doubt, as you also perceive, that she did anything intentional, exactly.

But, "innocent" or not, beautiful women know their power. And we all see the reaction we get from men. And secure, good women avoid ANY chance of appearing as if that reaction could ever be reciprocated in circumstances like yours.

Let me pause to say - HE - is the person who's 100% in the wrong. Him. Screw him for ever indulging in his mind... your sister. Jesus, your SISTER. Utterly terrible.

But your current thought revolve around understanding and approaching your sister, the sort of 'other woman.'

So, let me point out two kinds of women. I'll use personal examples...

My cousin (same age and people call us twins, but there are differences). She's GEORGOUS. Striking dark hair, big blue eyes, dimples :), tan skin, very very cute. But she is not a kind, decent, or considerate person. She NEEDS every man in every room to want her - my man, any man, married men, doesn't matter. She doesn't just act normal, certainly doesn't downplay things, she EXAGGERATES, puts on a show to get the guy (even though SHE'S married). I know her better than she knows herself. She'll try to seduce any guy I date, or even any friend. Even from afar! One funny example of my awareness, this guy that's liked me for years but is only a friend (long story) and I were hanging out, she called me (states away) then wanted, of course, to talk to him. (She thought he was a love interest). I said okay, but first "prepped" him. Told him exactly what she'd say, how she'd act, her tone, including that she would be soooo flirty, etc. etc. As soon as he hung up, he was like YEP. You called it 100% to the T. Just frustrating. If you even THINK I might be interested in the guy, don't do your hardest to make him want you! When we go out, I'm a saint. She's married, yet begging for male attention. I truly don't get it.

Other type: Me. I've always been considered very good looking. Blonde, big blue eyes, thin, curvy, big boobs, great rear, fit soccer player (+10 other sports). Smart, masters degree yada yada. But I'm EXTREMELY considerate and cognizant of my actions and empathic. I have ZERO desire to make someone else's man want me. Zero. I'm constantly referred to as a "unicorn." Any man would go for my cousin based on looks and first impression, admittedly even over me, but she's insecure, high maintenance, drama-obsessed, cheats, pathological liar, wants to be taken care of, no desire to work a day in her life, even as a stay at home mom (kids aren't even home), she secretly hires maids behind her husbands back. - won't work a job, but still won't even vacuum a goddamn floor, etc. etc. They might be very interested in her, but once they know her, she couldn't hold a candle to me (yikes that sounds bad, yikes yikes, but true). I go OUT OF MY WAY to be boring (not saying that ppl should, but I do) in front of her husband. I can be very charming. I turn that shit off in front of him and any taken or married man. To support a bit... in college, my best friend was totally beautiful, hisoanic beauty, huuuuuge boobs, etc, but this girl would be so 'friendly' to your face like your best pal while she was actually banging your man. I really think she preferred taken men. Other girls loved her! I...somehow.... evvvvery girl was sooooooo threatened by, but am the most loyal person ever. I could never do the things she did. My bff thought it was sooooo funny that women hated me for no reason and trusted her when the opposite was appropriate.

Beautiful women know the effect they have. They can use it to 'take' and build their own ego, or they can be careful and considerate. Your sister did the former. (A bit of assuming, but I have some experience.)

Long winded. Some women, despite if you're their best friend or sister or cousin - they don't care to 'take it down a notch' in front of your boyfriend/fiance/husband. They see that that man is interested, yet continue on course. They aren't being naive either.

She could have been careful, modest, not tried to be soooo omg charming in front of him (giggle, twinkle eyes, and hair flip! [touches his arm] "Omg you're so funny!"). It's not her fault, totally. It's his. But damn if she's not totally aware of what she did, and damn if she doesn't know she played a speaking part in this play.

End of the day, he showed you his true colors. Thank God. Move on. But be very careful who you trust in your life. Insecure people are wrecking balls. It doesn't even have to (impactfully) benefit them for them to screw your life ... for just an ego boost.

You should start the conversation with your sister. Just have her do some talking. Then kindly, delicately discuss how it played out.