MG/YA, Sci-Fi/Fantasy, read 2003 - Boy is recruited into special task force who can travel parallel planes/worlds. There's a black space between worlds. Spoilers.

[Maggie]

The boy has a snake bracelet that he found once and it turns out that his dead (twin?) sister left the bracelet for him and the agency that recruited him thought he had the special skills but it was actually the snake bracelet and his ghostly sister's presence that they were picking up on.

He had a friend who went through the trainings with him and was lost in the black dark space between worlds for a while. The group that recruited him ended up being slightly nefarious I believe and his ghost sister got kind of creepy towards the middle. like obsessed with him maybe?