The little sister is in a relationship with a "standard" guy. He is handsome, perhaps, but aloof. He forgets her birthday. He scoffs at her hobbies. The protagonist (the older brother figure) witnesses this. He feels disgust, then rage, then a plan. The initial hook is not lust—it is righteous indignation .
Hana reached out, placing a hand firmly on his shoulder. "But that's not how family works. You’ve spent so much time being the protector that you forgot you’re allowed to lean on me, too. I’m not that little kid anymore, Kaito. I can help carry the load."
In return, she gives me a sense of purpose and belonging. She reminds me of what's truly important in life and helps me see the world through her innocent and curious eyes. She's my little bundle of energy, always keeping me on my toes and pushing me to be a better person.