An off-duty MTA bus driver allegedly killed his sister-in-law and stabbed another victim in a Harlem apartment before cops shot him dead, sources said.

The chaos erupted about 6:50 p.m. in a building on Saint Nicholas Terrace near West 127th Street when 44-year-old Ubaldo Gomez knocked on the door of the building superintendent’s apartment where his estranged wife was staying, sources said.

Gomez shot and stabbed his 45-year-old sister-in-law to death when she answered the door, sources said.

He then tussled with a 39-year-old man from inside the apartment who intervened, sources said. It’s unclear if the man is the superintendent.

When police arrived at the fifth-floor apartment they saw the two men “engaged in a violent struggle,” Chief of Patrol Fausto Pichardo told reporters at a Wednesday night press briefing.

Gomez was told numerous times to “show the officers his hands,” Pichardo said.

When Gomez refused to drop a 12-inch kitchen knife and tried again to stab the man, a police sergeant opened fire, according to Pichardo.

Gomez was struck in the torso. He and his sister-in-law were declared dead at the scene, police said.

The suspect’s 41-year-old wife was not injured.

The 39-year-old man was hospitalized and in stable condition as of late Wednesday, cops said.

Police recovered a Glock 9mm handgun and the bloody kitchen knife at the scene.

Additional reporting by Kenneth Garger