Chimera

A standalone sci-fi thriller novel written by Rob Thurman, best known for the "Cal Leandros" series. Stefan Korsak is a bodyguard in the Russian mafia who is searching for his long-lost brother Lukas, kidnapped ten years ago when they were children. Finally, Stefan finds Lukas in a sinister institution which has been performing genetic experiments on children, training them to be superhuman killing machines. But 'Lukas', who insists that his name is Michael and refuses to accept that Stefan is his brother, has no memory of his life and no knowledge of the world outside the institute. Now on the run from Jericho, the mastermind behind the 'special' children, Stefan has to win his brother's trust and keep them both alive.....

Tropes used in Chimera include:

Big Damn Heroes: Anatoly, who has been MIA for the whole book, showing up just as Jericho is about to kill Stefan and Lukas.
Big Brother Instinct: Stefan epitomizes this trope. Don't mess with his brother.
Bilingual Bonus: The book is littered with Russian words and phrases, even though the setting itself is in America.
Bittersweet Ending: Stefan and Michael both survive, but it turns out that Michael isn't actually Lukas, and Lukas died ten years ago..... Which means that Stefan spent ten years searching for him, but by the end of it, he did find a brother in Michael.
Blessed with Suck: Michael's ability to kill people just by touching them, as well as the other kids at the Institute.
Chekhov'sGun: Lukas's Mismatched Eyes. See below.
Complete Monster: Jericho. Also Lewis Vanderburgh AKA The Babysitter.
Creepy Child: Wendy.
Crowning Moment of Funny: Stefan walks in on Michael watching porn and is forced to give him The Talk.
Crowning Moment of Heartwarming: "For my brother." Made even more heartbreaking when you learn he isn't really Stefan's brother. Followed by Stefan telling his father that Michael is his brother and no one is to say otherwise.
Deadpan Snarker: Both Stefan and Michael, which makes for hilarious banter.
Do Not Call Me "Paul": Michael refuses to answer to the name Lukas, so Stefan comprimises by calling him 'Misha', the Russian version of his name.
Evil Uncle: Uncle Lev.
Expy: Personality wise, Stefan and Michael are exactly like Cal and Niko Leandros respectively, with the roles of big and little brother switched. Saul is also exactly like Robin Goodfellow from the same series.
Fish Out of Water: Michael, who's had little exposure to the world outside of field trips and the occasional movie.
Groin Attack: Michael takes care of pedophile Vanderburgh by cutting off blood flow to his testicles.
Healing Factor: Michael heals faster than normal human beings.
Knight Templar Big Brother: Stefan, who's been searching for Lukas for ten years.
Meaningful Name: The children at the Institute are all named John, Michael, Wendy or Peter - the main characters of Peter Pan.
Mismatched Eyes: Lukas/Michael has one blue and one green eye. Then again so do all the kids at the Institute.
Papa Bear: Anatoly. The people who harm or threaten his sons tend to... disappear.
Parental Abandonment: Stefan and Lukas's mother died when they were very young, and their father disappeared shortly after Lukas was kidnapped.
Psychic Powers: Michael, and the other kids at the Institute.
Tear Jerker: Michael Taking the Bullet for Stefan and finally calling him his brother. He Gets Better.
Teen Genius: Michael. Stefan teases him about it.
Twist Ending: See Bittersweet Ending above.
The Unseen: Stefan's girlfriend Natalie, who's referred to several times but never seen. Anatoly, until the end.