Rachel's sisters, Jill and Amy Green, made memorable appearances on Friends, but it was never the original plan for Rachel to have two sisters. Reese Witherspoon played Jill in season 6, while Christina Applegate portrayed Amy in season 9. There were rumors that Witherspoon did not return due to tension with Jennifer Aniston, but those rumors were denied by both actresses.
The show's creators wanted Witherspoon to come back in season 10, but she was pregnant at the time. Both Witherspoon and Applegate remember their roles fondly, and playing Rachel's sister on Friends was an important role for them.