Joan Larkin is the oldest sister of Maryellen Larkin.
- Stan Larkin: Father
- Kay Larkin: Mother
- Carolyn Larkin: Younger sister
- Maryellen Larkin: Younger sister
- Beverly Larkin: Youngest sister
- Tom Larkin: Older younger brother.
- Mikey Larkin: Youngest brother.
- Grandpop: Maternal grandfather
- Grandmom: Maternal grandmother
- Jerry Ross: Husband
Joan is seventeen at the start of the series and finishing her last year of high school, "nearly all grown up." She finds Maryellen messy, klutzy, sloppy, and tomboyish. However, she still cares for her sister, and doesn't begrudge her attitudes. Maryellen thinks of her in part as a finicky fussbudget. She sometimes calls Maryellen with rhymes to her nickname, such as "Ellie-jelly" or "Ellie-deli."
Joan is very cross and snappy if she is woken up too early.
In the Books
Joan is at the all family meeting with the rest of the family. Upon seeing Maryellen's grass-stained shorts, Joan inches closer to Carolyn. She wonders where Betty and Florence will sleep when they come to spend the night and teases Maryellen that they can't sleep in hammocks; when Maryellen suggests the new room arrangements, Joan initially is against it due to Maryellen's messiness, but does offer to try. At the beach, Joan sits on a towel reading and does not get in the water.
During Maryellen's paint fiasco, Joan is inside getting ready for a tennis date with Jerry. Shen comes out to find what's going on as the brownies are burning, and her opening the door knocks Maryellen over into the bushes, and red paint goes all over. Joan yells at Maryellen for getting paint all over Jerry and "ruining" everything, and causing disasters and being a baby who needs to grow up. She and Jerry then go out, but she is holding Jerry gingerly to keep the paint off her tennis dress.
That night, Joan fusses at Maryellen tossing around. She then lets Maryellen know that she shouldn't be so hard on herself, because not all her ideas are mistakes; her suggestion that Jerry should ask Joan to marry him led to Jerry "pinning" her, which makes her engaged to be engaged--but she doesn't want Maryellen to tell anyone else yet. She also thinks the All Girls Room is a good idea and they should keep it permanent.