Desperate Sister

My sister 23 is used to getting everything she wants, spoilt as the only daughter since birth. I'm the younger of her brothers and older by 20 months. But my sister overplayed her hand you could say, letting her grades drop and expecting it not to matter with our parents. Well, it did matter a great deal with them and for the first time in her life. They cut her allowance and restricted her movements until all her grades greatly improved. Science was her worst subject and half her grade would come for the project she presented. Leaving too late to do a good job, in desperation she asked to help her. I've always been good at science and had old projects stored in the basement. Some earned me high marks and others were first ideas, I changed my mind about and done something else. So, it would be quite easy to update one of those and she could present it as her project. But what was in it for me, I thought to tease my sister a bit before agreeing. But her response first shocked me than aroused me. She offered me sex, neither of us were virgins. I was between girlfriend at the time and my sister is a gorgeous woman. I got a hard-on just hearing her say I could fuck her if I done a project. Recovering my senses I wasn't looking like agreeing and so she offered me 3 fucks. So, I agreed and asked for a fuck before starting and two after finishing. She agreed and we fucked in the basement on the old lounge that was stored there. We both enjoyed the sex, and then I looked over my former projects for one that would do good enough for her to pass the test. My better projects all had gotten great marks but were well known at the university. I chose a simple project based on wind power and solar power pumping water up an incline. It was a simple idea but was complex in construction. I'd only to clean it of dust and service the moving parts paint over my name paint in my sister's name/ I knew she would only get good marks for it not great marks. But if it was too good the professor wouldn't think she had done it by herself and mark her down instead. It was good enough to get her enough marks with the written test part to get B+. My sister had done what our parents wanted and got her allowance back and restriction and eased her movement restrictions. Not to be fully removed till she showed she could keep her grades up permanently. But my sister kept her promise, and I got to fuck her twice more. Well, she really had no choice, as there are more tests in her future including Science ones. So, I might get more sex unless she greatly improves in science between now and then.