Season 9: Episode 200: The One with Rachel's Other Sister
Chandler convinces Monica to use the good china dishes for Thanksgiving. Joey forgets he's supposed to be in the Thanksgiving parade with the rest of the Days of Our Lives cast, so Phoebe teaches him how to lie to his producer. Rachel's other sister (Amy) shows up unexpectedly for Thanksgiving and is upset when she finds out Chandler and Monica get Emma if Ross and Rachel die. And Chandler is upset that he only gets Emma if Monica is alive. Amy can't get Emma's or Phoebe's names right, carelessly insults everyone, and in general drives everyone crazy.