A madman killed his grandfather and wounded two family members — including his 4-year-old sister — in a machete attack inside Brooklyn apartment building on Sunday night, police and sources said.

The three victims were targeted in a second-floor apartment at a building on Putnam Avenue near Nostrand Avenue in Bedford-Stuyvesant at about 6:20 p.m., police said.

The attack was believed to be related to a domestic dispute. Dayquan Dubose, 22, first slashed the little girl’s right leg, according to law enforcement sources.

Dubose then fatally stabbed his 79-year-old grandfather in the stomach, before slashing his 49-year-old brother-in-law to the right hand, sources said.

Dubose’s grandfather died at a nearby hospital. The two other victims were hospitalized in stable condition.

A witness was able to hold Dubose until police arrived, sources said.

The suspect was in custody on Sunday night.

Additional reporting by Joe Marino