Kai should just back off here, his brother needs to learn his own lessons the hard way. He made his own bed, let him sleep in it. Shion is too good to be two-timed on like this, it's not worth it.
Sure poke your nose into other people's business all you want but don't start getting invasive and doing thing under the pretext "it's for a noble cause/it's for them/it's to help them" then regrets when they don't want/need your "help". The same critique I've got for most NTRs as well.
Here's one of the example that has great art to make up for it:
Really hope he doesn't accept this bullshit deal. Also why was he considering break up with his current girlfriend if she's so compatible and he planned to go long term? Guess he's not serious about anything.