Thing got out of hand with sister

I had Sex with my sister while she had a divorce

My sister wasnt happy with her marriage and got divorced. While this was going on she lived at my parents house. She is a blond young woman in her 20s and i was just 2 years younger. She was very attractive and i had jerked off to her quite often. So here is how it started. We had a Family gathering for easter and we were in the living room talking about old times when she checked for old VHS below the TV in a low Board. She was on all fours and her ass just looked to good in her tight jeans. I got horny and at this view i would never care If she was my sister. I went over and gave her a slap on the ass because it was just to inviting. She screamed and said i should Stop. But she did not got Up. So i slaped her again saying that she shouldnt present her ass like that then. Which she responded "im your sister" ....so i heard that but was looking at her ass and just said nothing but decided to hit her juicy ass one more time. But i was aiming this time. I wanted my middle finger going down her crack and slip over her pussy. So instead of going for the cheeks i went for the middle with a wide spreaded hand so my fingers would touch it all. She screamed my name while i retreated and sat down to hide my boner. This time she stood up looked at me angry and said again "im your sister" and i responded "all i saw is ass, i would hit it again If i dont see your face" which she just said "pervert" and left. But this wasnt over. Later the day i saw her bent over again. She was peting our cat and i got horny again just seeing her like that and immediatly went for it again. This time for the middle directly. " Uhhf, Wtf?!" Was her response while turning around. While i just Said "sorry was just to good" -"hello?! dont do that!" and that was it for the day. The next day she wasnt wearing a jeans but a skirt.
My parents were all over the house so we got time together again. We had a stool for our Couch that could be used alone or as an Extension.
My sister decided to lay over it while watching TV. Exposing her ass again. This time i got a real boner. I was sitting behind her in the Couch and you could Imagine all kinds of things down under the skirt.
The skirt was short but not to Short to see anything. I was edging my boner a little thinking about what to do. but sadly she stood Up then to go to the toilet. Later the day i was pranking her by following her around which she thought was quite annoying but i also did it to watch her ass. I had a boner the whole time and was edging and close to cum when i followed her to the kitchen and while she was geting a Plate i stood directly behind her. I was wearing a Jogging pant and my boner was looking out of it but was hidden under a big t shirt in top. I was edging to much and decided to go and jerk of but before i could do it my sister said "you are stupid stop following me now" and pushed her hips backwards. I wasnt going to move and kinda Catched by surprise. Her ass touched my boner, i know we were fully clothed but still i felt the cheeks warming my boner. We stood there for a second, probably because she was surprised to feel a hard boner too. But then she screamed "move!!" And started thrusting back and foreward with her hips. It was the second thrust that lined up to perfect my boner was lined up to her crack and then i had to move backwards grunting. She must have thought she hurt me somehow because she Said "Yeah that what you get" while leaving me bent over a little. The truth was, i was pumping a load into my shirt. I was so embarassed. I actually just came by a thrust from my sister. I left to clean myself and get a new T-Shirt. But this wasnt over yet, Just when i came Back into the living rooms she was below the TV again. My parents werent home and her ass was naked...her skirt was rolled up and she was just there in all fours silenty showing pussy and ass. Without saying anything i pulled my dick out got close to her WE could both hear each other breaving. This was it. I just ramed it Inside her. Thrusting wild and blow a load in her pussy. We were heavily moaning and then i stood up and left for my room. I was comfused and scared what would happen next. But actually later the afternoon when my parents were back my sister acted total normal like nothing happened and so did i. I have No clue If this is a thing now or a one time Thing. But when WE looked at each other WE both knew what we did but it seemed to be a secret now without words.