Failing Sister

I'm 27 and doing fairly well career and relationship wise. But my love life isn't anything you could call a standout. I do get sex, but I'm greedy and wanted more like most men. But my girlfriend worked shifts and it meant either her or I were working while the other was off and it restricted our Love Life somewhat. As I said I've a good career and had to study to get where I am. My younger sister partied a little much and was having trouble in her courses. So, she approached me, I told her to study more. But she needed help and knowing me like she did offer me sex. Neither of us were virgins or not active sexually. I was sex mad in my younger years and still am now. So, I agreed, I help her with studying, she gave me regular sex. Her grades improved and my balls were drained more often a win for us both.