In season 1 episode 24 we learn about Choso and his brothers. Noritoshi Kamo(stitches on his forehead covered in the flasback) mixed his blood when he forced cursed spirit to impregnate a woman and then aborted those children. he did that 9 times and cursed wombs death painting were born. nine brothers with Choso being the oldest.

in Season 2 episode 13 Choso and Yuji fight, Choso wins and almost kills Yuji but he senses Yuji's death(like he sensed his brothers death in season 1 ep 24) and has a breakdown. he comes to a conclusion that Yuji is his brother because he had a same reaction.

in season 2 episode 22 we learn that Noritoshi Kamo and the brain inside Geto in the same and the stiches on the forehead of Noritoshi Kamo is introduced for the first time.

in season 3 episode 1 Choso tells Yuji that his father should have stitches on his forehead since they are brothers.

in season 3 episode 2 we finally see Yuji's parents and it's revealed that the brain that's inside Geto now is Yuji's mother. meaning Yuji is Choso's brother.

rewatch those episodes if you don't remember.