Yk how tanjiro has redish hair, the opposite colour of red is green and Gyutaro has green hair.
Nezuko had black hair and the opposite of black is white, she also has different coloured tips, so does does Daki who had WHITE HAIR-
Their backstory where they got help and stuff is alike, the whole older brother younger sister bond, even how in the anime, their facial expressions were the same in some scenes.
This makes me so sad how Gyutaro and Daki turned out, when they seem like they could've been like Tanjiro and Nezuko...