The detail that introduces the central idea that the two cyclists might be related is found in lines 17-18.
The taller cyclist, upon hearing the shorter one's claim to be Edward Cruttenden, responds with surprise, stating, "The deuce you are! Why!--my name is Edward Cruttenden!" This revelation creates a sudden dim sense of their own similarity.
The shared name, Edward Cruttenden, becomes a crucial detail suggesting a possible familial connection or relation between the two cyclists. Their astonishment and the subsequent need to differentiate themselves before venturing into the world together add an intriguing element of mystery and potential kinship to the story.