Responses
Shoui and Kotoha are brothers because Yukino (Kotoha's mother) adopted Shoui, thus Shoui becomes the brother of Kotoha. They are not brothers by blood to be exact
Shoui and Kotoha are cousin/uncle and nephew. Because their fathers are brother and Shoui's mother are aunt of Kotoha's mother.
Shoui's father is younger brother of Kotoha's father.
Shoui and Kotoha would they be more like uncle and nephew..as Shoui is Uta's son and Rikichii's nephew..then Kotoha is Yukino's son and Rikichii's grandsoon ..but they can be cousins too as their fathers are brothers..(⊙…⊙ )