POLYXENA by Ailsa Zheng

Nine years into the Trojan War, fifteen-year-old Polyxena catches the attention of Achilles when he kills the brother standing next to her, Hector.
 
They meet three times in secret. A romance begins to bloom, but Polyxena is cautious. She despises her brother Paris for starting the war by choosing love over his family, and wants to be nothing like him.
 
Eventually, Polyxena finds out her brother Paris has laid out a trap. Armed with a secret that can bring Achilles down, Polyxena must choose a side who will live: the brother she does not love, or the enemy she does.