Melody and Harmony are twins in a society in which procreation is both a necessity and a competitive sport. I don't even remember which twin was which. Melody is melodramatic and self-centered, but infinitely more interesting than her poorly-written Christian zealot of a sister Harmony. 
 Harmony, Harmony... I myself am a Christian, and know of different types of Christians. And they don't talk like Harmony. Her speech was just...too much. Every couple seconds she was saying things like "Hallelujah!" and "This is a sin" and "Oh my gracious!" It was so stereotyped as to be something of a farce. Her characterization was flat and based on over-generalized views on religion. 
 If this is aimed at younger kids (and I mean kids; this writing is so juvenile it would to be boring to a teenager), this is what the author is saying to them about religion. It's really irritating that her writing of Harmony came off this way (all Christians are different McCafferty). 
 But perhaps worse is the subject matter. I mean, this is teen pregnancy we're talking about. However, it's not about how bad of an idea it is, but how much fun these girls are having bumping and being preggerific. It's about monetizing and commercializing it. The writing style and subject matter are too disconnected to really appeal to either younger audiences or older ones. 
 Still, though, it's really interesting. I want to see what happened in the next one. Hopefully Harmony is a little more bearable.