Context:

Sorry for the spelling mistakes, but English is not my first language. I know Im an asshole, but I wanted to get this off my chest.

I (F25) have a relationship of 7 years with my boyfriend. My sister has a relationship of 3,5 years with her boyfriend (32M). My sister and I are really close together, like best friends.

• Before my sister had a boyfriend:

When I met my boyfriend 7 years ago, he had a nice friendgroup. I became part of them. While I was in that friendgroup, I met my later to be sisters boyfriend (I call him Matt). He was single and had a nice character.

A great part of the friendgroup is the parties that we have together. We get really drunk and have fun playing (drinking)games or whatever. In the friendgroup we have open conversations about sex and stuff. The drinking games we play involves questions about sex and fantasies. Matt was always interested in my sexlife and the answers I gave. When we went outside for a cigarette, he always wanted to know more about me and my sex life and i was curious about his. Every party we flirted a little. For example, he would jokingly say he wanted a threesome with me and my boyfriend or asked if I was horny. Not something serious or anything. I had a boyfriend after all.

-Matt meeting my sister:

Like i said before, my sister and I are really close. I decided to invite her to a party with my friendgroup. Everyone liked her and she got part of the same friendgroup too. One day, my sister told me she had a secret relationship with Matt. She was scared to tell me because he was a good friend of mine. I was so happy for her and Matt. They both never had a serious relationship before and they were really cute together.

-The party a year ago:

This party I got really drunk. Matt was outside and I wanted to smoke a cigarette. We talked about sex and he asked if I've ever made nudes of myself. I was drunk so I showed a picture of me in nice lingerie (not naked). He said that I had a beautifull body. After that, he also told me that I had beautifull breasts and that he got a little exited when she saw me one time came out of the shower and saw my nipples through my shirt. That party we teased eachother a little and said that if we both didn't have a relationship at that time we wanted to have sex with eachother just to know how it feels. He said that I was " the forbidden fruit". Nothing happend that night ofcourse.

• Last party

We got really drunk again. At the end of the night we went outside, and I sat on his lap. I started touching his leg to tease him. Then he walked to the toilet, I needed to pee so I followed him. When he was done, we saw eachother in the hallway. He kissed me unexpected and he said I made him crazy. Yes, I kissed him back. The whole french kiss. We both said it felt good and wrong at the same time. Then we got in the crowd again and started dancing. After that, we were going for another cigarette. We talked and went a little bit further. We touched eachother above clothes. At the end of the night when everyone left the party, we went outside for the last cigarette (my sister, Matt, my boyfriend and I). The moment we stepped inside the house, I whispered in Matts ears :" Wait for me, I need to go to the toilet". When I got out of the toilet, both of our partners already went upstairs to sleep. Matt sat on the couch. I walked to him and sat on his lap and we started to french kiss again. Really passionate with hairpulling and stuff. He unhooked my bra and started feeling my breast. After that he told me I was a good kisser. A few times I thought I heard someone coming downstairs. I was really scared my boyfriend or sister could see it. He told me he wanted to see my body, so I started to take off my bottom clothes. I said: "Now is your turn". So he took his pants down. He grabbed my hand and placed it on his dick, and I started jerking him off slowly. He said he really wanted to eat me out. I said: It is so wrong we shouldnt be doing this. He agreed. We went outside for the last cigarette and he took me to the shed. Once there, we started kissing again and we wanted more. He took my pants down again and he started to touch me and eating me out. I stopped him and said that it was really wrong. I didnt want to hurt my sister or my boyfriend. I said that I never wanted to lose them, and that I was the one who could lose them both. I told him that it was purely sexual and that I didnt want a relationship or anything. He said it was the same for him. We stopped because we were both feeling a bit guilty. The whole time we said that it felt so wrong.

Moments later, my boyfriend walked in. He wasnt suspicious or anything. Luckely Matt and I stopped in time. We all smoked a last cigarette. (This time really the last cigarette) and we went upstairs to go sleep.

The next day we were all hungover. I asked Matt if he had any regrets. He told me it felt mixed. I told him I also felt mixed. We liked it, but it was so wrong. That day we went outside multiple times to smoke, but we didn't touched eachother of said anything about that night. Like it never happened.

The worst part is that I really want to have sex with him now. I think he wants it too, but it can't happen because we never want to hurt our significant others and my sister.

I secretly hope that the next party will go the same. It is wrong and I feel guilty, but at the same time it was really nice and exciting.