This is not something that she is willing to let go, so she decides to teach her big sister a lesson. Wearing only a tank top and her diaper, she tells Alex that she is going to learn how it feels to be in diapers. And, since she was such a brat, she's getting a spanking first.
Alex is not so tough once she's over her sister's knee, and she is soon kicking and struggling as she gets her shorts and panties pulled down for a bare bottom spanking.
Once her bottom is red and sore, Red makes Alex lie back and puts her in a diaper. Alex is so embarrassed and humiliated by the experience, but it isn't over yet. Red takes some pictures of Alex in her diaper, and threatens to send *those* to everyone unless Alex tells everyone in school that Red doesn't really wear diapers and that the picture was staged.