Season 10: Episode 221: The One Where Rachel's Sister Babysits
Monica and Chandler ask Rachel to write a letter of recommendation to their adoption agency; Joey feels left out because they didn't ask him, so they do. After a botched attempt to write a good letter, he does a re-write and drops it off without showing it to them first. The adoption agency thinks it's wonderful that they had a "child" write their recommendation. Phoebe unknowingly ridicules every idea Mike has for proposing to her. He finally does it the right way - he gets down on one knee and opens his heart. On Rachel's advice, her sister, Amy, breaks up with her fiance (whom she doesn't love), and temporarily moves in with Rachel. Rachel wants to help Amy be a better, more responsible person, so when Emma's nanny gets sick, Rachel decides to let her take care of Emma for the day. Emma ends up with pierced ears, and Rachel realizes Amy is beyond help.