Endgame by Dale Mayer

Tandy’s every instinct told her to jump in and to take over the care of her two half siblings. They were twins, born to her mother later in life. Tandy hadn’t been in a position to help her other siblings, a guilt that still ate at her. These twins, however, had special needs, and she wasn’t sure she was up for the challenge. Nonverbal, they appeared to live in an alternate world, one she didn’t know how to reach.
Keller worked alongside Grant in the FBI and was considering how his own growing psychic skills could be better utilized, when he was asked to help on a case that had very unusual elements. These were elements that Stefan and Dr. Maddy hadn’t seen before—and that was saying something.

Not until he meets Tandy does Keller realize how unusual and potentially sinister this case truly is …