"The One Where Chandler Can't Remember Which Sister" is the eleventh episode of the third season of Friends, which aired on January 9, 1997.
The guy living above Monica and Rachel has removed the carpets on his floor, and he's too loud for the girls to stand. Phoebe tries taking a stand, but as Monica already pointed out, he's too charming to intimidate, and Phoebe starts going out with him. The girls also hear her having sex with him in his room upstairs, and leave their own room in a hurry.
Rachel finds her new job at Fortunata Fashions is just serving coffee. She vents her frustration with the wrong person - classy chef Monica, in her fake boobs at the Moondance 60's diner - but she runs into the right person - Mark, who manages to get her an interview at Bloomingdale's and coaches her through it. Ross is not at all happy about this and fears that Mark only wants to have sex with Rachel. She lands the job, to Ross's dismay.
Meanwhile, at Joey's birthday party, Chandler gets too drunk from vodka jell-o shots and fools around with one of Joey's sisters, Mary Angela. Joey is pretty mad at Chandler for groping his sister until he's deceived into believing that Chandler may really fall for her. Things get out of hand, however, when he goes to talk to her and is, once again, in the presence of all of the sisters; he even mistakenly kisses Mary-Therese, believing that she's Mary Angela. When Chandler can't recognize Mary Angela and apologize to her, Joey gives the go-ahead to his sister Cookie, who gives Chandler a black eye.
At the end of the episode, Phoebe apologizes to Rachel and Monica for the fact that they had to listen to her having sex with the man upstairs, but insists that she has really developed feelings for him. But then they all hear him having sex with another woman. While Rachel and Monica comfort Phoebe by telling her that he "sounded like he was having more fun with her" an irate Joey goes upstairs to give him "the butt kicking of a lifetime," followed by Ross and Chandler. But bizarrely, he's somehow able to calm them down with a single sentence when they confront and start to berate him.