The victims were discovered inside the apartment on Sherman Avenue in Inwood just after 9:45 a.m.
Police responding to a 911 call found blood seeping out from under the apartment door.
Authorities say the victims, ages 51 and 37, are roommates and were both found unconscious and unresponsive.
They both had multiple stab wounds on their necks and chests and were pronounced dead at NYC Health & Hospitals/Harlem.
A knife was recovered at the scene.
Kelvin Valdez, 27, was taken into custody about a block away for questioning and was later charged with murder and criminal possession of a weapon.
Police sources tell Eyewitness News that there is a domestic violence history between the suspect and the younger of the two victims, Jaslynn Pena.
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The brother of 51-year-old Belkis Cruz said his sister owned the apartment and was renting a room to Pena. He said she had moved out from living with her boyfriend, who he said is the suspect.
"The boyfriend of the other woman is who killed my sister," Vinicio Cruz said in Spanish. "I live on Long Island and I couldn't believe what I was hearing when they called me. My own sister. Couldn't believe it."
He says his sister found love later in life and was planning on getting married.
"Never in my life, during the many years that I've lived here, have I seen something like this happen," the victim's neighbor Guillermina Ferreira said.
The investigation is active and ongoing.
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