My Sister, Benedict Arnold is the fifth episode of Season 3 of The Brady Bunch, and the 54th overall episode of the series. Written by Elroy Schwartz and directed by Hal Cooper, it first aired 15 October 1971 on ABC.
Greg and Marcia use their dates to get back at each other, and the younger Brady siblings prepare for the school carnival by building a dunking tank.
Marcia is excited that a high school boy has asked her out on a date. Marcia tells Greg about the date and wants his opinion about the boy. Without yet telling him who the boy is, Greg tells her that the only boy who he would object to is his school rival, Warren Mullaney. Marcia's date happens to be with Warren, but based on Greg's comments, she figures that she will still go out with him since it is solely Greg who has a gripe with Warren. She however plans on not telling Greg. Greg does find out that Warren is Marcia's date, and he forbids Marcia from ever going on a date with him again.
Although she was never planning on going out with Warren again because there was no "spark" between them, she decides to ask him out again solely to spite Greg. However Greg brings home Marcia's rival, Kathy Lawrence. Marcia and Greg clash over this and Mike admonishes them for using their dating partners to make a point. They go to the other room to apologize, but return to find that Warren and Kathy have taken a fancy to each other and left together.