Police probing the April 24 rape and murder of a 9-year-old girl in Madhya Pradesh`s Rewa, has found that her teenage brother sexually assaulted her after watching a pornographic video on his mobile and then killed her, following which his mother and his two elder sisters helped in the cover-up, an official said.

The police cracked the case and detained the victim`s 13-year-old brother, their mother and sisters aged 17 and 18 after interrogation of 50 people, intense questioning of the accused persons and on the basis of technical evidence, he said on Saturday.