Seigi no Mikata

Seigi No Mikata ("Ally of Justice") is a Josei manga by Chiaki Hijiri.

Pity poor Youko, who's spent 17 years being tormented by her demonic older sister Sumiko. As our story opens, she suffers her latest humiliation when Sumiko tells Youko's hot new boyfriend that she suffers diarrhea, causing him to dump her. But this is par for the course for a woman who cheerfully maces the neighbors dogs, and permanently scares off all the patients at the health clinic she's just started working at. Youko can only hope and pray that one day, Laser Guided Karma will finally hit its biggest target.

But what's this? Youko's new boyfriend was planning to dump her as soon as he got some sex? And the dogs were terrifying the neighborhood? And the clinic was operating without a license? Why does Sumiko keep getting civic awards and recognition in the newspapers? Could she actually be an... ally of justice?!

Tropes:

 * Asshole Victim: If whoever Sumiko torments doesn't end up benefiting from her behavior, they'll usually turn out to be this.
 * Decoy Protagonist: The manga is from Youko's point of view, but she's not really the main character.
 * Disproportionate Retribution: Sumiko's specialty.
 * Karma Houdini: Karma appears to be actually reversed for Sumiko: Her vicious pranks and acts of petty vengeance often end up as good deeds.
 * Omniscient Morality License: Sumiko is this trope incarnate.