Actress will play Mary-Louise Parker's estranged older sister.
Leigh will appear in at least two episodes of the series, which got picked up last year for a fifth and sixth season.
When Nancy (Parker) finds herself and her family in danger, she sends brother-in-law, Andy (Justin Kirk), to take her child, Shane (Alexander Gould), to stay with the older sister, Jill (Leigh), that she doesn't know that well.
Then Jill finds herself not liking her own "suburban family" and she reenters Nancy's life.
The fifth season of Weeds comes on Showtime in June.