@All my friends are fake so I started reading to cope myself Biologically they're not cousins, Yorrichi never had any children but he gave his earings to the Kamado family (Tanjiro's family) to carry on his legacy, making Tanjiro Yorrichi's successor. Muichiro's family is directly related to Yorrichi's brother (the Uppermoon 1 demon) making Muichiro the soul heir of their family. You can probably consider them cousins if you want