Friend rape my sister but I helped him do it.

This all happened on the day of my friend's birthday were he was turning 27. I'm 26 and my sister is 19 and was always the flirty type with guys but never wanted to commit to any sort of relationship which is kind of annoying for men. Anyways my friend and I roomates in a condo because we both work and still go to school so it makes it easier to pay the rent. So that night for his birthday we were going to celebrate it by getting drunk and play games, but that same night my sister wanted to crash in our place for a couple days. I did tell her beforehand that we were getting drunk that night and she said she didn't mind. Around 7pm we finally start getting wasted taking some Jhonnie Walker shots and drinking some other whiskey and before long we are completely drunk. My sister was also partaking in getting wasted and starts to flirt with my friend egging him on and my friend starts flirting back. It keeps going like that until he tries to kiss her and she backs away telling him she was juts messing with him. predictably he got really mad and asks a huge favor, I ask him what does he want and he tells me to roofie my own sister so he can fuck her. I do hesitate but I he reminds me I owe him huge favor for what he did for me a while back. So I give in and put a roofie pill in my sister drink and 30minutes to an hour later she passes out on the couch. Next thing I see is my friend gets my sister naked and starts fucking her hard and keeps going like that for 15min until he starts cumming inside her pussy. He then asks me if I'm going to fuck her too and I told him no, not really into incest. Then he picks my sister up and tells me that hes going to keep fucking her for the rest of the night cumming inside her until he is completely dry. Next morning we all wake up and my sister is walking around like nothing happeded so I asked him what happened so he proceeds to tell that after he had his fun he cleaned her up and put her clothes back on and says she remembers nothing. He made me promise to keep this a secret forever.