Family & Friends

How would you react if your best friend suddenly made out with your sister?

So my best friend invited my sister to a party we were going to, And then i jokingly told him not to fuck my sister, and he promised me not to. At the party my friend and my sister were really drunk, me not as much. Now my friend kept touching her and hugging her, and keeps telling me "its just a hug". After the party me my bff my sister and another friend go to me and my sisters house, when we went inside i was talking to the other friend and then went into the living room were i catched my bff making out with my sister on the couch, i pulled my bff away and we went to my room were he said im sorry, now i went out to brush my teeth and when i got back into my room my boyfriend wasn't there. So i asked my other friend if he knew were my bff was, he didn't know, so i just knew that he had gone to my sisters room with her. I took my friend with me upstairs to my sisters door and we could hear kissing from the other side. I knocked on the door and we went inside. As we went iside they immediatly stopped i picked up my bff and asked my other friend to follow him to his bed (my other friend wasn't as drunk either) I know had a long chat with my sister and she just said she was drunk and asleep all the time. The day after when my bff went home he apparently texted my sister he wanted a friends with benefits type relationship with her, and he texted her this after he send an apology message to me. he also apparently deleted the messages to my sister after she rejected him. This was now 3 days ago, and today i had a talk with my him and ended up forgiving him, even though i knew if my sister would have said yes to his message, he would probably have prefered her over me, and he didn't keep his promise when i asked him to not hit on my sister. Did I do the right thing here, and how would you have reacted. (everyone in this story is 17 btw). And what do you think about my way of writing (im practicing for my English exam)
How would you react if your best friend suddenly made out with your sister?
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