On a rainy day, Gretel bullies Bruno into telling her about Shmuel; Bruno lies and says Shmuel is an “imaginary friend.” Here, the author uses idiom and verbal irony to show Gretel’s dismissive attitude toward her brother’s playmate:
'He sounds like a barrel of laughs,' said Gretel. 'I wish he was my imaginary friend.'
The idiom "a barrel of laughs” is normally used to describe someone exceptionally entertaining. The phrase implies that such a person contains a huge volume of entertainment within them. However, the phrase carries a verbally ironic meaning here. Gretel employs the phrase sarcastically to suggest she actually thinks the opposite is true. As she uses it here, the phrase implies that Bruno’s friend must be dull or unimpressive.
This sarcastic use of a typically positive expression is sisterly meanness. She’s making her mockery of Bruno even more dismissive than it would be otherwise. By saying she thinks Shmuel sounds like a “barrel of laughs,” she’s trying to make Bruno feel bad about how he entertains himself in their lonely life at "Out-with." Her words directly contradict their literal meaning in a way that makes them acutely cruel. This irony shows Gretel’s dismissive attitude and her lack of empathy for Bruno. She often treats him as though she knows far more than he does and belittles his lack of experience, and this moment is a prime example.