Ignoring the incest (the obvious elephant in the room) i have no reason for why he even comes to like her. He witnessed the accident so what, is not like he ever gave af, why would he now. I'm all for weird ahh plots but this one is just too wishful
My guess is she started to change. Her behaviour, her actions towards her brother and her pranks were hard to predict since he believes everything is boring the minute he was born. Her unpredictability and her actions towards him annoyed him but piqued his interest. So when she got hit by the truck, he had a realisation that the reason she gets on his nerves was actually because he had started to"like" her (▰˘◡˘▰)






Please update
Everyday I pray that whenever I open mangago, it update
You and me both girl