Lies, damn lies and lies you tell your sister

During a layover in Chicago, I overheard a fragment of a conversation between two flight attendants. One asked the other in an incredulous tone, "Why would you lie to your sister?" Now I've never had a sister, but immediately I could imagine an infinite universe of reasons I would lie to her if I had one. Indeed, I cannot imagine a single situation that would call me to complete honesty with a sister.