Overheard Phone Call

I 23 was sitting by the window slightly behind the curtain reading a textbook studying for an upcoming exam. When my sister 21 came into the room talking on her cellphone, to who I don't know. But she said something that got my full attention, she mentioned she got really horny seeing erect penises in photos and in videos. She said if she saw one in real life she would fuck the guy straight away. I decided to test her statement; our parents were going to the golf club for some trophy night dinner. I knew my sister liked to enter our father's home office when our parents were out. Not really expecting her to actually fuck me but maybe get her to masturbate me at least. She of course was on her cellphone when our parents left, I went into my father's office stripped naked and started slowly stroking my cock. It when hard, I was unsure how long my sister would take before entering the office. But for once her call was brief and she entered catching me mid-stroke. She was stunned at first at seeing me naked, but she recovered and couldn't take her eyes off my cock. Nothing happened, so I asked if she wanted to stroke it for me, adding she was looking at it like she wanted too. Without saying a word, she came over to me and started to play with my cock. It had been going down, but it recovered its hardness straight away. As she played with it, I decided to feel her up and she didn't try and stop me. With her allowing me to feel her up, I drew bolder and put my hand inside her panties and started fingering her. Still no push back from her, I asked her to let me fuck her. She just nodded and we went to my bedroom, where I took my sister's virginity. I took great care not to leave any sperm inside her and since then I've fucked three more times and showered with her about seven times as well.