Larry's Sister is the twelfth episode of Season 7 of Three's Company, and the 140th overall episode of the series. Written by Paul Wayne, and directed by Dave Powers, it first aired 11 January 1983 on ABC.
Summary[]
When Larry's teenage sister Diane comes to visit, Larry asks Jack if he'll show her around, since Jack is the only guy Larry trusts. But Larry's role as an overprotective big brother gets way out of hand when he discovers Jack and Diane are both gone at two in the morning. Since the note Jack left on the bathroom wall fell off, Janet and Terri also believe Jack was taking advantage of Larry's sister.
When Terri discovers the note, the girls quickly apologize to Jack. But after all, going on a grunion run did seem a little unbelievable. At any rate, the girls have their work cut out trying to get the two best friends back together. After hearing the truth, Larry still insists that Jack apologize first. Janet, knowing Jack's stubbornness, finally gets an idea when she sees Jack angrily chopping onions, with tears streaming down his face. She tells Larry Jack is "too broken up" to apologize. Touched, Larry takes the bait and apologizes first.