Circle inspector Kishore Babu said that the accused K Subramanyam went to his father-in-law's house last midnight and hacked to death his father-in-law V Venkateswara Rao (55) and his (Subramanyam's) wife V Vijaya (25) as everybody in the house was asleep.
V Sivaramakrishna, the brother-in-law of the accused, woke up hearing cries of victims and tried to rescue them.
The accused attacked him and his mother Sarojani too. Both were seriously injured.
Venkateswara Rao and his daughter Vijaya died on the spot. Sivaramkrishna was battling for life at Eluru government hospital.
Family dispute is suspected to have led to the murders, police said. Probe is on.