Why? Do you even know what it means in the context of the story? She doesn’t mean it in any other way other than “big brother.” There is no implied romantic feelings to their relationship, nor has he shown to have any feelings for her other than seeing her as a little sister/daughter type. As a matter of fact, even Aina’s mother mentions how she herself confused him for her husband because he looks so much like him. I can see how Aina got so attached to him from the start, it’s not like she can start calling him dad/father, now that would be weird. In other words, you really need to open your eyes and see a story for what it is. It’s a wholesome story, nothing else and you’re missing out on it with your nearsightedness. Good luck.
I stopped the moment...I saw..."onichan"!!!