My sister tried to get my husband to cheat on me with her and I don't know how to go forward from here.

My husband (31M) and I (28F) have been a couple for around 8 years and we've been married for almost 5 years now. We welcomed our twins into the world about 7 months ago after an unplanned and rocky pregnancy. Things have not been easy for my husband and I after the twins were born either because I was hit hard with PPD and he was still working full time to keep us above water financially. My mother (53F) my two sisters (21F & 33F) have been helping out with the twins and keeping my mental health from imploding... I'll admit that I was struggling for some time before my sisters noticed and confronted me about it. But they have all been so much help and we couldn't have done it without them.
Recently I started working again with minimal hours to give my husband a break and more time at home with the twins. Meaning that my mom and sisters would sometimes be alone at our place for short spans of time, but overall it was going to be easier on everyone's schedule, and gave my husband and I a whole day to be alone with the twins (something we hadn't had in a while).
Something that I didn't realize is that with the new arrangement, my older sister would be alone with my husband when he got home from work. This wouldn't be a problem except that she has blatantly flirted with him for as long as I can remember, especially when alcohol is involved (getting too close to him, touching him, paying a little too much attention to him, comments that she makes...). She's also mentioned to me, at least once that she thinks my husband is "so hot" and that she thinks that I'm "really lucky to have landed that." I've never had a serious conversation about this with her because I would've liked to think it was something that she didn't mean anything of/ it's always been something that has been easier to ignore... My husband has also been convinced that it's harmless/ just her trying to get a reaction out of me. Anyways, they were alone a couple times and each time my husband said that everything was fine, that he would get there and my sister would be nice, but she wasn't being too friendly with him. Then yesterday he called me at work asking if I was busy/ when I would be home, and when I got back he was pretty upset, and he told me my sister made a move on him.
He said that it was the same as every other day that he got home, they said hi, then he went to our room to get changed out of his uniform and check on the twins. She followed him into our room, asking him about his day and he said that she kept getting closer to him until he had him almost against the wall, then she told him that if they had sex, she wouldn't tell anyone if he didn't... and proceeded to rub against him, put her hands on his genitals. He shoved her away, but he said that he froze initially... But he said that the encounter ended with him telling my sister to get out, and then she apologized and left.
She texted my husband later, asking him to keep what happened a secret, saying that she's been going through a lot lately and she didn't know what she was thinking, also that she doesn't want to create a rift between me and her because she made a mistake, also that she doesn't want to stress me out right now. He didn't reply to her, and I texted her saying that we need to talk.
My husband apologized over and over for putting himself in that situation, he's not handling it well, he's mad that he froze, he's mad that he didn't realize that she was flirting with him, he's feels like he betrayed me in some way... which honestly brings me back to my own assault and a lot of the feelings I went through. To say the least, my husband and I are pretty ****ed up emotionally right now.
I don't even know how to handle this... I'm so mad at her. Do I let her tell her side first? What do I do if she tries to tell me a different story? I believe my husband and he deserves someone to stick up for him, none of this was his fault... If anything I blame myself, I didn't intervene when I should have and now he's paying the price. I want to cut her off, but it feels wrong to do since she's been going out of her way to help with the twins, and I haven't even talked to my mother about this yet, and I'm afraid that she's going to tell me that I need to work this out with my sister
TL;DR - My sister who has a history of being a little too friendly with my husband tried to sleep with him. I'm having a sit-down with her in a few days and I need advice on how to handle the conversation with her.


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