sick and tired of being sick and tired

It wasn't sudden ss the story unfolded, he remembered that his brother always wanted him, always liked him.
He didn't want to be better than him at first, he just wanted to follow in his older brother's footsteps.
This gradually got into MC's head, until he finally started to like his brother.
Of course, this doesn't justify his brother's horrible behavior, but if you're reading this story, you certainly weren't looking for a green flag.
Let's not be hypocrites; we all know what we're getting ourselves into.

Yeah, I get it. I knew what I was reading and I was aware of who obviously was going to be endgame who was going to end up being a plot device for the main pairing. I just wanted to vent out my frustrations. (￣へ￣)

Seeing it in that lens definitely feels a little less.. sudden.. but in the beginning it felt as though mc had that one daydream where they fucked and then next chapter he started to become all flustered and seeing him in a new light. It was in the later chapters that’s he began looking back on their childhood and his brothers past behaviours and then started feel even more fluffy about him iirc..

Please correct me if I’m totally off though, I read this at like 3 in the morning yesterday and I’m having a hard time remembering certain scenes

You need to read that again, honey, haha.
And he was flustered at first because he didn't understand why he was having those thoughts, he was embarrassed.
Being flustered doens't just mean you like someone, it could be anger, sadness, shame...