You are a senior in High School (last year for those who don't know) and your sister just started. She becomes the target of several older girls who bully and tease her. Your father remarried so she has a different surname (her fathers) so no one knows she is your (step) sister. She asks you for help and the two of you plan a humiliating (and sex-filled) revenge. The girls all have a huge crush on you. You choose which girl or girls and what you do to them. Pictures are taken with them in extremely compromising situations to use for blackmail. They become you and your sister's complete slaves to keep you from releasing said pictures. At first they are unhappy but quickly become aroused and enthralled by the perverted things you make them do. Soon they are you and your sister's personal sex toys as they beg you to use them in any way you wish.

Personally, I'm in favor of any high school / college setting story. You have my blessing

Really? Did you have a look here and there?

(Sorry for the self advertisement, but I thought that it might be good to connect these ideas for the sandbox option in a future version of XSP.)