Let's say that I have a cute 2D imouto, hypothetically speaking. And this cute, adorable, perfect-looking 2D imouto is trying to wake me up by using the classical attempt: Riding Onii-chan's Dick At 7 In The Morning to Wake Him Up... Attack!
As you can see in the hissatsu's name, the correct way to address me is with onii-chan.
But then again; let's assume that she's one year younger than me, and she just wants to ask me something. How should she address me then? I'm not into a master-slave roleplay, so onii-sama is out of the question. Nii-san sounds too plain and has no impact at all. We're not training under the almighty eye of our grandfather, deep in the mountains, somewhere in Hokkaido, so aniki isn't an option. The only choices left are nii-nii and nii. While nii can deal a lot of moe damage if she combines it with puppy eyes, it might still fail if I, for some reason, can't look straight at her. The only logical and most effective choice to deal the most moe damage and conquer the onii-chan route is to call me nii-nii in a sweet voice that directly results in an one-hit K.O.
Thank you for your understanding. Imouto characters are the best, unless they're tsundere.
This is so accurate that it deserves a price.