The basic premise of Kenka Banchou Otome (the anime, anyway) is that Onigashima Hikaru (a boy) and Nakayama Hinako (a girl) swap places, resulting in Hinako attending Shishiku Academy (a boy's school) while disguied as Hikaru.
Most of the main cast has never actually met the real Onigashima Hikaru prior to the swap, and both Hikaru and Hinako are sort of androgynous, so the deception works.
But Hikaru's elder brother Houou also attends Shishiku Academy, and encounters Hinako (in the guise of Hikaru) multiple times, starting in episode 9. For some reason, the disguise fools Houou as well - he shows no signs of being aware that the person he's speaking with is not actually his brother Hikaru.
Is there a reason that Houou is also fooled by Hinako's disguise (other than "the plot demands it")? We can infer, based on their conversation in episode 9, that Houou and the real Hikaru have met before (they weren't raised separately or whatnot), so this is rather odd.