Most doujins satisfy this requirement and more than half throw in Snake for good measure.
>select all the fish
Well, captcha, if you insist, I'll bury my face in their fish
Tough choice but I'm going to the right.
The Gaen sisters are better than the Fiyah Sisters.
Koyomi is the MC, and the series is portrayed through him. If he doesn't consider them to exist, they don't.
On the other hand we could infer a more objective reality with clues and hints from other characters.
That makes way too much sense to be correct. Aside from the general water thematic the Gaen/Rainy devil users share, just the fact they're diametrically opposed from an ideals point should make them Water Sisters by default.
This is some Schrödinger's boyfriend shit right here.