The Wind Hashira, Sanemi Shinazugawa, and his brother, Genya, have a strained relationship. This is because Genya once accused Sanemi of murdering their mother, though he was only protecting Genya after she had become a demon. Sanemi is very hostile toward Genya following this, and no longer considers him his brother and even threatens him when Genya tries to make amends.
During a break in his training, Tanjiro witnesses first-hand the hatred Sanemi carries for Genya. In addition to brushing off his brother’s apologies, he tries to gouge his eyes out when he learns Genya eats demons in order to become stronger. Despite Sanemi’s position, Tanjiro cannot just stand aside; he pushes Genya out of the way. Though Sanemi confirms he isn’t trying to kill his brother — that would be against the Corps’ code — he admits he wants to injure him so badly that he would have to leave the organization. This results in a long-lasting brawl between Tanjiro and Sanemi, which has to be broken up. Tanjiro’s part in the fight is not without consequence. He is banned from approaching Sanemi, and by extension, from the training course that the Hashira had set forth.
It is disappointing he cannot take part in that trial, but Tanjiro has proven that he is plenty strong enough. It is even more heartbreaking that he is unable to reason with Sanemi and restore his relationship with Genya.