Choso and Yuji Itadori are indeed related – they are half-brothers, thanks to Kenjaku’s manipulations. Namely, as Noritoshi Kamo, Kenjaku was responsible for the creation of Choso and his siblings. Much later, being in possession of the body of Yuji’s mother, he also “gave birth” to Yuji Itadori, which makes Choso and Itadori half-brothers, despite the age difference being quite large. When Choso realized this, he stopped treating Yuji as an enemy and joined him in defeating Kenjaku.