I won a overseas Trip

I'm Robert 25 single and I won a trip overseas for two. I didn't have a girlfriend at the time. My sister Rachel 22 want to go, then I found that the prize was for married couples no singles. We decided to pose as a married couple and we when on the trip together. The ten day trip was in Europe and we would have to share a room. As it turned out we were the youngest couple on the trips tour. Also all the rooms we got were only had 1 bed, this didn't worry us and we gave each other space. We couldn't flirt with anyone, but we had fun anyway. On the fifth night there was a banquet for the tour and we both had too much to drink. Back in our room and I walk in on Rachel changing for bed I want to pee. She was topless and said You go ahead and use the toilet. I took out my cock and peed, then Rachel removed her panties to finish changing. After I had peed and seeing Rachel naked I got a hard on. Rachel seeing it said You got a hard on looking at your sister. Yeah would help me get it to go down I said, How she asked. Anyway you want I replied. You want to fuck your own sister do you she asked. I reckon I do I answered. Okay lets fuck she said. We had sex twice that night and the next day we were happy about what had happened and had sex again that night. Rachel was on contraception so no worries about babies. We had sex every night after that first time and still have sex together now six months after getting home from the trip. It has become a casual thing between us and we date other people, but like to get together for sex now and then.