I know most people will think I am the villain of the story which even if I am I don't care one bit. I am writing this just because I want to know whether I am justified in not regretting it. So I'll give some background on this situation. Give honest feedback.

I am the oldest of 3 siblings. I'll use fake names because I have my cousins on my pain account. I will call myself Terry, my youngest sister (F31) Clara and my younger brother (M29) Ronnie. Ronnie was always the golden child our parents always loved him more then us. I was definitely the least favorite. To give you an example: for our birthday's Ronnie got the biggest cake, Clara would get a medium one and I would get absolutely nothing because they forgot my birthdays half the time and I stopped celebrating them. Clara was the only one who actually loved me she always on my birthdays would get me a cupcake which she would save from her pocket money.

I am the least favorite because I was an unplanned pregnancy. My mom got pregnant at 16 and was forced by her and my father's family to have me. They hated me since I was born. I for my younger years was raised by my Aunt May and Uncle Ben (Yes it's a spider-man reference) I could count the number of times my parents told me they loved me since my delusional past self wrote it down as "evidence" that loved me god I was delusional. I had to take on loans for college while my parents paid for Ronnie's. Speaking of him he is an entitled bratty bi*ch. He believes everything should be handed to him. We as kids just us siblings went to the mall once and he disappeared for 20 minutes and we found him in an ice-cream shop and rather then apologize he just said "pay up" I left him there and didn't come back. That slap and beer bottle to the face which gave me a scar was so worth it. Ronnie got nothing no screaming no slaps no nothing. Clara got the occasionally slapped screaming and our father when really angry or drunk I would remove her pants or pull up her skirt or dress and spank her. I got slapped, punched kicked and even had bear bottles thrown at my face.

Let's get to my Sil I'll just call her Cassie she is my ex. We met when I was 33 and she was 29. We met at a cafe where she worked and she kept flirting with me the entire time I was there so I thought why not considering she was really pretty. We dated for 2 years and I was considering marrying her. I came home to take her out to the cafe where we met to propose but when I came to our apartment I saw my brothers shoes which was weird because they weren't close. I heard noises from the shower and when I peaked in I saw them together. Cassie was in her bra and panties and Ronnie was shirtless and in his boxers. I know that isn't considered "true sex" but considering his hands were inside her panties it would have escalated. I then fully barged in and it was pretty weird yelling at my brother and girlfriend basically naked in the shower but I straight up yelled at them for like 10 minutes. Ronnie looked annoyed at least Cassie looked ashamed.

I asked how long you know what they said? 6 months. They told me they were in "love" pathetic. I kicked her ass out of my apartment. My parents were furious... at me! Cause I just couldn't get over my brother sleeping with my girlfriend for 6 months. I went even more low contact then before the only reason I am in contact is for my inheritance does that make me selfish I don't care. I was and kinda am depressed about that. I haven't dated anyone since any strand of trust I had left. They got married after 1 year of dating. That selfish entitled bi*th had the nerve to ask me to be a groomsmen since he didn't have much friends. Of course I didn't attend got harassed for that as well

Fast forward to my ex is quite unhappy in her marriage because from what I know the thrill of the affair is gone and I guess she doesn't love him but she is staying for the stability he provides which is kinda funny since this is the reverse of the situation when the affair happened. I had gone to the family beach house for a vacation since I had nothing to do. The rest of the family went on a hike but me and Cassie didn't since we both are afraid of heights they were gone for 6 hours. That lead to the current situation. We were watching a TV show. She then brought some wine and we started to drink quite a lot and led to a conversation between us. She started to cry saying how she hates her marriage and she is only still married for money. I laughed at her telling how this is the reverse of the our previous situation.

Then it led to this conversation:

"I really miss having your dick in me." She said in tears.

"Even if I hate you I still miss pounding you to the ground." I replied laughing.

And then she came close to me touched my dick and kissed me and I kissed her back. Then I pushed her on her back and getting on top of her and we kissed for like a minute and even though I hate her having those lips touch mine brought me joy. I removed the panties from my skirt pulled my pants down and pounded her (which felt great) we went on for 2 hours were completely naked by the end of it. Luckily we both got over the beers by the time the rest of the family came out. I really enjoyed have sex after 3 years. That was 2 weeks ago and me and Cassie have slept together twice once in her car and in my apartment. I told Clara about it and she told me that she doesn't support my actions. She told me she wouldn't snitch on me but what I am doing is "disgusting" so that is making me think am I wrong? I mean he did this to me why should I care about him? So yeah AITAH?

TLDR: AITAH for sleeping with my Sil who is my ex after my brother did the same thing? Give me honest feedback if I am worse then idk Joker then tell me.