In the anime it's not mentioned why Inari's brother Touka can see the gods, and even they seem to be surprised that he can see them without having divine powers. Is there any explanation why he can see the gods in the manga?
In the anime, there is a flashback scene where Touka gets lost on the Inari mountain and Uka-sama transforms into a human so as not to scare him and guides him home.
You could assume that's when Touka became aware of Uka's existence, remembered her divine aura and could see her ever since. Or she granted him the vision of divine creatures, since he can not only see her, but other gods.
There is no explanation in the manga (yet) either. However, Touka is not the only one who can see Gods, some people living in the shrines can see them, too. It just seems to be a coincidence that Touka can see Gods, too.