SciFi 2 kids with enhanced DNA grow up, save humanity by building space arks

[Charles]

I remember reading this in Walmart around 2009ish. It was about two kids, brother and sister perhaps? They grow up together, but far advanced due to enhanced DNA (Alien/genetically modified somehow?), and fully aware so they hide their gifted nature from everyone else. They eventually lead a movement to build a fleet of arks to send into space in an effort to save humanity from a doomed Earth.

[Michele]

do you recall how old the kids were, or why everyone was leaving Earth?