I very vaguely remember the series but it was about a boy who almost died saving his sister from a flood. A water-type goddess saves his life but at the cost of her very high "ranking" and it's up to the brother/sister duo to restore it back to its former glory. The sister is well-known around the town for thinking that she is the recipient of the water goddess's powers but it's revealed that it was the brother instead that got her abilities. The last I remember of it they were trying to keep what happened a secret from their mother and got busted.
There are three other characters involved that I remember: an older deredere type sister who has a secret crush on the older brother, a younger sister who also has a crush on him, and a cat-goddess or something that's kind of perverted (where only the boy - and maybe the goddess - can tell that it's not a normal cat).