Inside the little sister mansion.

Onee-chan.

Little sister mansion = A place where the little sister creatures (!?) are found. They refer to the protagonist as "Onii(Onee)-chan!"

The toilet in a little sister mansionlittle girl's house!!?

How... could I not drink the water?

It wouldn't be Elona if you couldn't drink the toilet water.

And if you're lucky, you can get another wish here.