Being sick and stuck in bed since he was a kid would definitely mess with his mental health. You can just imagine how lonely and hopeless he feels because of his condition, so he might had thoughts like if Yuzuru finally thinks for himslef and stop being his robot caretaker he'll be left completely alone, but he also feels guilty for dragging his younger brother into his misery. The parents didn’t really help either by dumping such a huge responsibility on the younger brother when he was so young. All in all, he's a bit contradictory, but these brothers just care about each other so much. What I can't get over tho is how easy the mother just got away with everything. Didn't even apologize
I thought he was adopted
Sometimes the brother seemed like he wanted yuzuru not to worry about him and live for himself, but sometimes it seemed like he wanted to control yuzuru and keep him tied to him. It kept bothering me and I couldn't focus on much of the story because of it