Well, seeing as todays audience will mainly find it disgusting as your sister did, you .. might want to up her age a few years. Not necessarily needed, but a thought.
Now, as per her childish spirit, does it contrast the other character? Is he more down to Earth, more mature? It might be a good underlining for the old phrase 'opposites attract'; however, while Egyptians married, they also had more than one spouse. I don't know if it was monogomous, if there was divorce, or if they simply moved away from each other, or what really.
I also don't know how exactly they were married. Was it love? Was it purchase? The best person I know to refer you to would be Nicole Hanson at GlyphDoctors http://www.glyphdoctors.com/
, as she has studied the prospect a little more in depth than my just going off of what I learn from the Beeb and books. The easiest way I can think of to contact her is to register, click on her image on the front page (she's the pretty smiling woman) and click the little 'Contact' link at the bottom of the page.
Now, as for if it's 'too young', you need to worry about the audience here. Are you aiming for whoever has a bit of an interest in Egypt? Do you want to target an audience who has some background in the field, and would know of the times? Moreover, realize that while marrying someone in youth is frowned upon in our Western culture, it is more frequent in Eastern and Islandic (I wonder if that's the word...) cultures (e.g. Russia's marriage limit is 14).
For ideas, here's what I have off of the top of my head without knowing much of the background of the book: The man can't afford the wife - The two realize their differences and wait until she gets older (perhaps tie in the man 'cheating' on the girl) - The two fit perfectly dispite their differences and live happily - Someone dies.
I don't know how much help I was, but there were my two cents. Good luck!