As she seized up, I told her to clench her arse tight and then release it, and, as she did so, I gently pushed the tip in and, with great restraint, h...ld it there for a while. Then I told her to clench it and release it again and this time I slid right in. “Wow! That’s actually exciting” she exclaimed in a rather muffled voice as her mouth was otherwise occupied. I couldn’t see exactly what Cathy was doing but it took no more than a few minutes for Tracey to come, pushing Cathy’s face into her. My eyes were focused on the couch and not on either of them but I noticed Mom and Sis look at each other then Mom looked back in my direction. Mom said, "Honey, it was decided that we should share", she said.I looked up at her with a blank stare and said, uh huh" as I continued to look at her while waiting for clarity.I glanced at Sis who had a smile on her face then she rolled her eyes, looked at me and said, "She knows.....WE KNOW !".The blood felt like it drained from my body as I went numb.. We didn't need to really. She was more than keen enough to let me watch her strip on cam, and she masturbated freely. Except we didn't use the m word. She was wanking, or showing off, or frigging herself. Or doing as she was told, which was a phrase she particularly liked.So there was no doubt about whether she was for real or not. No doubt about whether she was turned on or not. No doubt, judging by her range of toys and the way she showed them off, that she wasn't shy.So the next step had a. The view was beautiful; it’s raining heavily with thunders and lightening. The cold of the weather, the heat of my sister’s body giving me a different feeling inside. I was standing so close to her keeping my face on her shoulder. Few minutes later my small innocent penis was a hot wild monster. I slowly grabbed her arms and rubbed, passing warmth in her as I saw her shivering because of cold. I touched my monster on her back. She stood stunned. Not a single word from both of us.Still trying to.