Responses

I'm honestly shocked at how everyone's interpreting his actions as romantic towards a child. They've either never cared for children themselves or never had someone care for them (I.e. older brother, cousin, family friend or whatever). If they did and they interpreted these actions as romantic towards a child or towards themselves as a child then I would be very fucking concerned
I'm not sure why everyone is having an issue with their ages. Right now, there is zero romantic feelings. The only one who has feelings is the kid, and that's only because he is still mentally an adult. Taehyuk is just acting like an older brother or cousin, or as if Hangyeol is the younger sibling of a friend, hell even just someone looking after a kid, and if you see his actions as anything other than that then you're the issue.
Also I can understand the brother finding it weird but his behaviour is also fucked up. He doesn't truly care about Hangyeol, his actions are pure narcissism and all about control. If he truly cared about his brother, he would've reported Taehyuk to the police, considering he is a minor and it is a weird situation.