The Agreement

I'm Reagan 29 single average build and was called a nerd at school. I did really good at school then university and ended up with a great job and career. My only sibling Monica 26 was struggling with university and was endanger of failing her subjects. She was a cheerleader in high school and liked to party a lot. She scraped into university and found it harder than she thought and she just couldn't concentrate on her studies. Our parents told her she had to pass all her subjects or they would cut her off financially. She just got thru her first years at university, but the final year was harder and she was failing all her subjects. I still live at home like Monica and when our parents were at a friends wedding last year. Monica approached me to help her pass her final year subjects, I told I would help as much as I could. But she wanted a lot more than that, she wanted me to get a high pass mark in her subjects. This would mean I wouldn't have any spare time for myself and I would have to help do her assignments and anything else she needs doing. I told her I would help her, but she would have to do the study and I couldn't do her exams for her. Then she offered me her body as an enticement saying she would be my sex slave and I could do anything I wanted to her once she passed. I said it's a great offer, but after you pass you could say no. I'll let you have sex with me while you're helping me study, but only basic straight sex nothing else she replied. We came to an agreement and I would get sex once a week from her up to the final Exams and anything I wanted after she passed. I told her I would have be shown what she liked at first. Every week we had sex right up to the final Exams, that week she was so stressed that sex was the last thing on her mind. I was thinking of what I could get her to do for me but also thinking she might back out of the agreement if it was too over the top. I decided om anal sex I knew she had ever done that before, we had done blowjobs and Sixty-Niners before. After the exams, we waited to see her results, she got high passes in all but one subject. But she still passed it just. After everything had calmed down and started to return to our normal routine, I wait for her to tell me when she would honor our agreement. As the weeks went by I was starting to think that she wasn't going to honor our agreement. Then our parents went on a weeks holiday leaving us home alone. The first night we were alone, Monica said I ready to let you do whatever you want to me. I said I would like to try anal with you if you're okay with that. She replied I'm your sex slave and you can do whatever you want with me. I replied I don't want to hurt you and if you don't want to do anal I will do anything you want. If you take it easy and don't rush I think I can handle you fucking my ass she answered. With that, we went to our parent's bed and had sex normal sex first then we tried anal sex. In the morning we showered together and she said she would let fuck her every night till our parents got home. She even let me fuck her ass a couple of more times.