Humour and imitation are important parts of child development. The affiliative nature of these behaviours allows children to communicate, connect, and strengthen their relationships. Sibling relationships are uniquely involuntary and offer children the ability to be silly and imitate one another. Additionally, play is a rich context in which we can observe and learn about children’s use of these affiliative behaviours. In a sample of 65 4-year-old children (n = 65, M age = 56.4, SD = 5.71 months) with an older (n = 28, M age = 75.8, SD = 11.2 months) or younger sibling (n = 37, M age = 34.9, SD = 5.3 months), the following study investigated use of humour and imitation during naturalistic observations of free play with either an older or younger sibling. Based on prior work, dyads’ production of humour and imitation were coded when instances occurred individually or simultaneously. A novel behaviour called “humorous-imitation” was coded when humour and imitation occurred simultaneously. Positive responses as well as laughter in response to humour and imitation were also coded. Humour and imitation were both very frequent during sibling play sessions, occurring more frequently individually than simultaneously. Siblings sustained reciprocal conversations while producing both behaviours. Children responded positively and laughed in response to the production of humour and imitation both alone and together. Overall, this study contributes to developmental literature demonstrating that humour and imitation are important affiliative behaviours in childhood that should be encouraged together, particularly in the context of play and social relationships.