This house I moved in to has a water heater that is on it's way out. Luckily I'm the first one in the shower in the morning, so it's only an issue for me about the last 2 minutes of my shower. Princess (being a kid) should have no problem with showering at night.
Me: when was the last time you took a shower?
Princess: uh, I don't remember
Me: that means you need a shower
Me: there just isn't enough time in the day to explain why
Princess: I like to take my shower in the morning
(an obvious avoidance tactic that will not work here)
Me: there isn't enough hot water for 3 people to take a shower in the morning
Princess: then why don't YOU take a shower at night?
Me: because contrary to what you think, you are in fact not a real princess, this is not your castle, and sorry, but we are not your royal subjects
Princess: a look of confusion
There are ways to get around the incessant "why" questions my friends. And trust me, they will come. You have to be a thousand times more annoying back to get rid of them. It takes practice. But it's worth it.