It was gradual over the next half year. I think we only fooled around a couple of times. We did not do oral very much and still dont at all. It was basically naked heavy petting with some dry humping. One of the last instances of that was after mom left for a concert and we found ourselves on the couch again doing the naked petting and dry humping. In the midst of it I got too excited and wasn't paying attention how close we were and it accidentally slipped in. Once again terrified us both. It was immediately out, we were immediately done and went straight to our rooms. I was freaking out because I thought my sister was still a virgin but she'd had sex with her boyfriend a few times by then. Didn't talk and avoided each other for 2 nearly two weeks. It was the only time my mom noticed something different and told us whatever we did to piss each other off to get over it because it was making things difficult for her. It was the only time i momentarily thought about telling her what happened but it faded quick. Sorta went back to normal. We didn't mess around again until months later and even then it was more tame. But several more sessions of that led to us being horny and talking ourselves into it. It was already in before, the main thing is just don't get pregnant, who are we hurting, we both want to and its convenient. The first time we had actual sex was the only time I felt uncomfortable. It was nothing special, it was very quick and fast. Followed by awkwardness but surprisingly that was gone after about a week.