The Staten Island teen accused of beheading his mom’s boyfriend allegedly showed his little sister the mutilated body — then asked if she wanted their mother to live.  “She said, ‘Are you gonna hurt mom?’ ” the suspect’s weeping mother, Alicia Zayas, said of her 16-year-old daughter Bri’s encounter with her blood-soaked son, Damien Hurstel, in the Monday horror. “And he said, ‘Do you want her to live?’ “And she said, ‘Yes, please.’ “He said, ‘Okay, she’ll live.’ ” Bri then asked her brother if