In my opinion, I do not think their background should be explained this early on, it would take too much away from everything else in the issue. The Inhumans are undoubtedly going to be involved with the Ultimate Fantastic Four within their title, but I would not want to see taking up the whole Annual.
An Idea for a semi-explained background, What if they were the Royal Family of Atlantis, and the cosmic event that caused it to sink under what is Modern Day NYC turned the royal family, and their associates, into the Inhumans. Crystal would be the first Inhuman -born- into inhumanity, and would thus be curious to reach out to the world around them.
I mean, what if there's this huge beautiful semi-destroyed city in the caves miles under New York? That'd be a perfect place for them, because if Crystal slipped out for a moment, she'd be able to meet Johnny. Didnt they have a thing for each other before the whole Quicksilver deal?