I feel like the brother is just trying to test Mihoro because he really cares for Chiei. I'm guessing he had an ambiguous relationship with his senpai who eventually ditched him for an older guy and got heartbroken so he doesn't want the same thing to happen to Chiei. Mihoro does keep staring at him a lot so he might be misreading that as interest and so he's worried.
....I will be really disappointed if this isn't the case. ?