In The One With the Inappropriate Sister(5.10), in which Rachel broke up with Danny because of his unusually “close” relationship with his sister, Danny asked Rachel if she had any brothers, and Rachel said that she did not, but that one of her sisters has an unusually “masculine energy,” to put it ambiguously. From the audience laughter, this line was apparently intended as a way for Rachel to coyly imply that her sister was somewhat butch, or masculine in some way. A few years later in The One where Chandler Can’t Cry(6.14), we met Rachel’s sister Jill, played by Reese Witherspoon. There is certainly nothing “masculine” about Reese Witherspoon, unless you change the definition of the word. Now in this episode, we meet her sister Amy, played by Christina Applegate, who made an entire career out of playing a slutty sex bomb on Married…With Children, and is about as masculine as a lemon-scented doily. So which of the two was Rachel talking about? We know that she only has the two, not only from this episode's title (If there were another, then Amy would simply be another one of Rachel’s sisters, not her “other” one), because Ross said that "neither" of her sisters were coming to her party in The One With Two Parties(2.22), she specifically told Ben in The One With the Truth About London(7.16) that she had two, and Monica told Rachel in The One With The Baby Shower(8.20) that she invited "both" of her sisters to her shower.

It really is a contriviance for the sake of humor that when Amy twice calls Phoebe "Emma," Phoebe just says, "Phoebe," which Amy thinks is just a weird noise she makes. It isn't very plausible that Phoebe--or someone else--doesn't simply say "Her name is Phoebe!"

Great Phoebe Line:
(After Monica had asked the gang to hold food in midair over their plates while cutting it because they’re using her wedding china, Chandler is crestfallen to hear that in the event of Ross and Rachel’s death, he and Monica would get Emma, but he wouldn’t get her himself if Monica died too…)
Chandler: “Well, what is wrong with me, a-am I incompetent? Because I managed to survive whatever it is that killed the three of you!”
Rachel: “Honey, you’re taking this the wrong way. W-We think you’re going to be a wonderful parent. It’s just…y-you’re more, ya know, the ‘fun’ parent.”
Ross: “Yeah, and we’d wanna make sure that Emma also has someone like Monica, who’s more of a disciplinarian, someone who can be firm, and strict.”
Monica: “That’s not how you see me, is it?”
Phoebe: (holding her food in mid-air to cut it) “No, you’re all about the fun.”