I get that but I'm also thinking, his parents must've been that way towards the older brother too, as an eldest, I honestly can't understand his behavior because I could never, but at the same time, I wonder how his childhood and teenager years were? How did he feel about his parents always been absent and throwing the responsibility of his little brother to him + their expectations of him succeeding. And if he was raised never receiving normal attention and affection, how can he know how to give it, this isn't an absolute rule but it's the case for many people. He might think since he handled it his little brother would too, he wouldn't understand his brother has more emotional needs and experiences neglect differently than him.
Idk man but if I were in MC's place I wouldn't be able to forgive his brother. I understand he found him annoying but not once did you try to hear him out and for all those years?! And kept on blaming him? And now that he knows he's not so 'annoying' anymore and got his act right he's giving him attention and care? I'm not only blaming his brother but also their careless/irresponsible parents!