Janet Adler is Grace's "screwed-up" older sister.
Throughout the series, Grace occasionally mentions Janet's bohemian lifestyle and her penchant for recreational drugs.
It is implied that Janet is in bad terms with her mother for using their Aunt May's urn for her stash of drugs. Grace mentions Janet once stuck her head in the oven after which they had to put her in a mental facility for three weeks. Janet also recalls that Grace once "lit a fire under [her] butt" when they were kids and that she used to shave her head because her boyfriend Tony told her that she's too full of herself.
Grace also recalls that her sister (presumably Janet) was so high at her wedding that she thought it was Grace's bat mitzvah and tried shoving twenties down her dress.
In 2004, Janet visits New York where Will invites her to stay with them until her jewelry line takes off. This infuriates Grace, telling them how sick she is of dealing with Janet's irresponsibility and refuses to take her in. Janet returns a week later transformed and has seemingly built her life together with a new job and a new apartment.
She invites Grace and her friends to dinner at her apartment, thanking her little sister for motivating her. Will and Jack applaud at how Janet has succeeded in such a short time, which again infuriates Grace, saying things are not the way they are supposed to be and that Janet is supposed to be the "screwed up older sister". Janet finally admits that Will orchestrated everything just so that Grace can have a normal older sister.
- She lives in a van.
- She calls her sister Grace "Smudge" or "Smudgie", although never explained why.
- She may have been named after Janet Eisenberg, the friend of Max Mutchnick from whom the character Grace is based.
- Janet was first mentioned in Lows in the Mid-Eighties when Will visits the Adler house during a flashback. However, the family deny that Janet exists, implying her acrimonious relationship with the rest of them.
- The title of the episode Janet appears in is a reference to film The Accidental Tourist which earned Geena Davis the Academy Award for Best Supporting Actress in 1998.