It is bad to fight bullying with bullying.
Kelsey has learned that her younger sister Deena has been bullying other kids at her school.
Kelsey wants to teach Deena that being a bully is wrong.
Kelsey tells their parents what Deena has been doing.
Kelsey figures out what Deena has being doing to the other kids and starts doing the same to Deena at home.
Kelsey's parents talk to Deena about whether she's going through anything and punish her appropriately.
Deena lashes out against Kelsey by bullying other kids even more because she feels like her home life is out of her control.