Genya is Sanemi's younger brother and only living sibling. After a Demon attack befell their family, Sanemi and Genya were the only ones who survived the incident. However, after being branded a "murderer" by Genya, their relationship began to worsen and decline until the two split and went their own ways. It is revealed during the battle with Kokushibo that despite the whole incident, Sanemi still heavily cared for his younger brother and actually wanted him to live a normal life without the threat of demons. Genya is moved to tears and apologized after hearing Sanemi's reasoning behind his rudeness. After their battle with Kokushibou, Genya apologized again for blaming Sanemi and thanked his brother for protecting him before succumbing to his wounds and crumbling into dust, leaving Sanemi stricken with grief.
Sanemi shows deep respect towards Kagaya, believing that he is the most important person in the Demon Slayer Corps who should always be first priority.
When Sanemi first met Kagaya, he did not respect him and had rudely assumed that he was just letting Demon Slayers die while he was sitting in the safety of the Hashira to protect, saying that his life wasn't the top priority. However, after Kagaya mentions his reasons and his very weak body, Sanemi began to understand. Sanemi gained more respect for him when he learned that he memorized all the names of the fallen demon slayers, including his friend who had died.
Sanemi was heavily angered when Kagaya had died in the explosion in order to slow down Muzan Kibutsuji.