Sister Used By Old Teacher Improve My Grade

So back when I was eighteen and about to finish school to leave for university I was called in to mr Johnson's office, he was an older guy well known to be Abit weird with girls but not withholding guys and all he had to say was that my English literature grade wasnt good enough for the universities requirements. Of course I was upset and when I told my family so we're they. But the next day when I reached school for the last of my exams but weirdly enough my older sisters car was in the car park. A little context, my older sister was always pretty. She had a good curvy figure with shoulder length brown hair and quite large boobs and an ass that turned everyone including my friend's heads. But she was at my school before she went to university and knew some of the teachers. She was 27 at the time and was a teaching assistant. As I made my way to class, I saw her talking with Mr Johnson before entering his office. I decided I could be late for school and went to peek after five minutes. I saw something shocking, through the window blinds I saw what was initially my sister's jeans and shoes now on the floor, and her sat on a seat facing mr Johnson while using her huge soles to rub what turned out to be his hung cock. While my sister looked reluctant Mr Johnson could be heard saying "mmmmm I missed this"
Soon he put her feet down and used his hand to lower her head to his cock to which I then saw for the next three minutes her head bobbing up and down his shaft. Eventually he used his old hand to push her away then gestured at her to which she bent over the desk displaying her arse and I saw Mr Johnson proceed to fuck my sister hardcore from behind! My sisters eyes turned up in reluctant pleasure until finally Mr Johnson pulled out and came all over her ass cheeks. I hurried to class after this and later on break noticed her car was gone.
It wasn't until later I was called again into Mr Johnsons office and he stated after deliberation my grade was satisfactory of which I was over the moon.
It's been two years now and apparently my sister is now a teaching assistant at that school, maybe Mr Johnson is regularly using her? Don't know but it turns me on to think so and would love to see if she does, even my nerdy students would be hot