Urgh... my grammar hurts ╮(￣▽￣)╭
Mmm, I see where you're coming from, but I think Yukiya needs him because he loves him, and always has. His personality is very serious and quiet, and innocent. He's loyal to the man who took him in and treated him like a son, and yeah, he wants to fulfill his father's last wish, but I think it's his wish as well. To take care of Hideomi, not just because father asked him to, but because he loves him. They were so close as children, and Yukiya didn't think that what Hideomi did to him was a bad thing, so I'm sure he really missed Hideomi after he ran away, and never blamed him.
Not at all, yukiya expressed how he also loved hideomi at chapter 3
http://www.mangago.zone/read-manga/jika_renai_chuudoku/mf/m_v-1-chapter-3/pg-7/
Idkw but I have mixed feelings about their relationship... I'm really into incest or step-brothers related stories but I don't really like Yukiya, his feelings seems really chaotic idk.
Hideomi loves him but Yukiya just "need" him that frustrating. I have the feeling that Yukiya just need his brother bc he want to fulfill his father last wish... Disturbing
(Btw I like the last words hahaha so perturbing, I like that)