The answer to that question depends on what you mean by “realistic fiction.”
If you’re asking whether the book has any science fiction/paranormal/fantasy elements, then the answer is that no, it does not. It’s completely realistic, in that the abilities that the Naturals have are simply at the extreme end of the kinds of abilities we see in the real world.
There is nothing “unrealistic” about what Cassie, Dean, Michael, Lia, and Sloane can do, any more than, say, what Sherlock Holmes does. They’re simply cognitive outliers, who—through a combination of genes and the environments in which they were raised—have some pretty incredible skills. Their abilities are based in real-world psychological research, which you can read more about here:
But if by “realistic,” you mean “requires no suspension of disbelief,” then I think it might be more accurate to call The Naturals “unrealistic realistic fiction,” which is a term my friend Ally Carter invented to describe her books about teenage spies and thieves. The NATURALS books are realistic, in that they’re set in our world and there’s nothing in them that’s even remotely magical, but they still require readers to be be willing to go along with the basic premise—in this case, the idea that the FBI has a unit that uses gifted teenagers to solve cold cases.