The bodies of Jasveer Kaur, 40, and her 15-year-old daughter Prabhjot Kaur were found by neighbours on Wednesday evening in a pool of blood on the ground floor in their three-storey house in Tilak Nagar area of west Delhi.
The woman's 17-year-old son Jaspal Singh, said to be the only witness of the case, was seriously injured.
As the teenager is still in critical condition at the Deen Dayal Upadhyay Hospital, the police investigation has been based only on circumstantial evidence.
"The incident came to light around 6.45 p.m. Wednesday after Jaspal somehow came out to the street and fainted. The neighbours went to his house after finding him critically injured," said a police officer.
"As Jaspal is still unconscious, we could not get information about the attackers. We are probing the case from all possible angles," said the officer.
Police said the two women were attacked on their throats, heads and other body parts with blunt and sharp objects.
They said the attackers probably entered the house between 4 and 6 p.m.