I (27f) have three children with my now ex Kon (29m). 10 months ago my sister (26f) confessed that she and Kon had slept together the week before and she said she regretted it and needed us to be okay. She told me it was a mistake and she got caught up in a moment because she had a crush on Kon and she needed me not to hate her for it. That she loved me and wanted us to make things work. Kon didn't even try to deny it. He told me he'd been sleeping around for years and that he slept with my sister more than once. My sister denied it strongly. I broke up with him immediately and told her to get lost. I said I never wanted to see her again and that she disgusted and repulsed me and I didn't care how bad she felt, I wanted her to feel bad, I wanted her to hurt and she would never hurt as much as I was right then so forget about me.

I got checked after I broke up with Kon to make sure he didn't give me anything with all the cheating. I was negative for all STDs and I'm incredibly thankful for that. With the amount of cheating he talked about I'm still surprised I didn't catch anything.

She ended up dating Kon because I rejected her. Or that was the story she told others. She said she felt guilty and needed some good to come from this because clearly I would never give her the time of day again. And that's true. I swore to myself that I would not forgive her. Even through therapy I have become more secure in the belief that what she did broke our relationship beyond repair and regardless of whether it was one time (which I don't believe) or a full affair it was wrong and we'll never come back from it. I really don't care what happens to her and if she died tomorrow I would not grieve or make an appearance at her funeral.

Recently my sister found out Kon had cheated on her and he didn't only cheat but he passed along some STDs/STIs. I don't know specifics. I believe she's also waiting to make sure he didn't give her HIV. Although I can't say I paid enough attention to know if that was a big concern or just them being cautious. But she thought that experience would make me more forgiving of her. She was wrong. She showed up at my place and I shut the door in her face without saying a word. She showed up at our brothers house while I was heading inside and he shooed her but she told me how sorry she was again and how scared she was and how much she needed me. I ignored her and kept going and our brother told her she wasn't welcome.

She tried again and this time I did respond. I told her I do not care and if anything she deserves everything she got after what she did and how stupid she was in the end. She told me she needed me and that she was scared. I told her just like when I found out my partner of 8 years and father of my children was cheating and I could have used my sister only she betrayed me as well by cheating with him. She said it was one time and she would have been there for me and I told her I didn't want her disgusting self to touch me. Because I said that's how I saw her now a disgusting backstabber. I told her I don't care if he's given her every STD under the sun, because I want nothing more to do with her and she can die in a hole for all I care.

I haven't seen her since. Apparently she's very distraught and she's also the topic of gossip in our social circles. She lost most of our shared friends and a few have shared a similar sentiment to me on her. But a couple have asked me if I don't even feel a little bad for being so cruel to her and turning her away a second time even after all her apologies. They said they felt like I should have found a way to work through this because we were close for such a long time and sisters are for life or whatever stupid crap they said.

I don't feel guilty. I don't even feel bad for her or have any kind of empathy. AITA though?