The entire time Mahiro interacts with Kaede has been literally behaving like an Onii-chan, even so far as to consoling her on her studies, asking about spending plans and starting with a bike of all things. And also being the snickering little thing to even record the nonsense she does to his own sister then sharing to Miyo.
Mahiro has been very perceptive of what everyone's been up to, despite being against social interactions.