It is about a girl named Martina, or Marty for short, who defies all stereotypes of little girls: she's active, doesn't wear dresses, and hates things which are too saccharine. She has a rivalry with her sister Melissa who, unlike her, is very stereotypically feminine.
The drama begins when the girls' mother wants to convert Marty's room (which she calls her "Marty Den") into a sewing room, forcing the girls to share a room, but when Marty accidentally knocks Melissa out, she realises she actually loves her.
The Worst Thing About My Sister provides examples of
- Accidental Murder: Discussed when Marty accidentally knocks Melissa out and is afraid she will die and that she (Marty) will be tried for murder. Thankfully, Melissa survives.
- Animal Lover: Marty loves all animals, from puppies and kittens (though not as "cutesy-pie pictures" on clothes) to porcupines and snakes.
- Birthday Episode: Towards the beginning, Marty is forced to attend Alicia's birthday party against her will.
- The Bully: Katie, Ingrid, and Alicia are all schoolyard bullies who tease Marty and call her "Bluebottle".
- Censorship by Spelling: When Marty doesn't want to reveal what her nickname is, her mother assumes it's rude and asks her to spell it out.
- Daddy's Girl: Due to the sisters' mother's grumpy attitude, Marty gets along better with her father and they like to hang out together.
- Embarrassing Nickname: The three bullies, Katie, Ingrid, and Alicia, call Marty "Bluebottle" for the blue dress she wore to Alicia's birthday party, which embarrasses her.
- Embarrassing Pyjamas: When Marty accidentally knocks out her sister Melissa and they take her to the hospital, Marty is embarrassed when she realises she's still in her pyjamas.
- Fat Bastard: Alicia is pudgy and rude, being one of the three girls to tease Marty.
- Her Codename Was Mary Sue: Marty writes stories and draws comics about a superheroine named "Mighty Mart", who she based off herself.
- Inelegant Blubbering: When the girls find out that they will share a room, they both break into undignified tears. Even worse for Marty, she gets called out for acting unreasonable, but their parents console Melissa, who's older, and tell her she's being brave.
- In-Series Nickname: Martina nearly always gets called "Marty". She and her father also call each other "Curlynob" because of their curly hair.
- Intentional Mess Making:
- Marty once pranks her Neat Freak sister Melissa by throwing her toys around their shared bedroom and writing, "I hate bossy big sisters with big fat bottoms" in lipstick on the wall.
- Discussed when Marty is pretending to feel nauseous to get out of going to Alicia's birthday party. Marty's mother tells her not to "be deliberately sick" on her dress, but that wasn't Marty's plan.
- Limited Wardrobe: Marty prefers to wear boots and a T-shirt that reads, "Pow!".
- Ms. Imagination: Marty makes up stories and plays make-believe with a collection of toys.
- My God, What Have I Done?: Marty feels intensely guilty when she accidentally knocks Melissa out.
- O.O.C. Is Serious Business: The first sign that Melissa is actually unconscious as opposed to just playing is when she doesn't respond to being tickled.
- Pink Means Feminine: Melissa is stereotypically feminine and wears a lot of pink.
- Playing Sick: Marty attempts to pretend that she's sick to get out of going to the birthday party. Her mother isn't fooled, though.
- Prefers Proper Names: Marty's mother is the only one to call her by her real name: Martina.
- Shout-Out: Marty and her dad like to watch Toy Story 3.
- Sickness Equals Redness: When Marty pretends to be sick, she draws on her face with a red crayon.
- Title Drop: Marty frequently points out what the worst things about Melissa are.
- Toilet Humour:
- Apparently, Marty's den used to have a stain on the floor from when she "made a pig out of [herself] with her birthday chocolates and didn't get to the bathroom on time".
- Marty's nickname she has when she hangs with her guy friends is "Farty Marty".
- Marty insults Melissa by saying she has a "big fat bottom".
- Tomboy and Girly Girl: The main premise of the story is that Melissa is stereotypically feminine, while Marty isn't.
- Weight Woe: Melissa worries a lot about getting fat.
- You Are Fat: One of Marty's ways of getting under Melissa's skin is calling her fat.
