Lil bro doesn't realise yet that it is part of her charms. He might do once he gets a gf... or maybe never since they are siblings and whatever she is doing would be annoying to him lmao.
[Several years from now when Keita and Yuu are married]
[Little Brother just walks up to Keita and puts his hand on his shoulder and silently and telepathically sends a "You've had it rough, huh?" message through Bro Communication to his new (legal) bro]