Draco Malfoy's Mate and the Secret Sibling

There was something that Dumbledore had failed to tell Harry that would've made his life happier, but he has a sibling he never knew he had, a twin sister named Hazel Potter. Why did Dumbledore lie to him when he asked for anymore family relatives? But also why does Draco Bloody Malfoy keeps staring at her, why does Hazel use sign language rather than talk?