The younger brother has become a 3rd wheel in his own brother's relationship. No doubt he will be an irritant right up to the very end of the series, and will probably try to make Ronron fall for him instead. Well, he's already started doing that. I can imagine him doing dirty tricks to get to be with alone Ronron such as sending her a text that the three of them should go somewhere, like a festival, but when she gets there he tells her that Tarou didn't want to come (even though he didn't tell Tarou anything about it).