A boy, 5, and the 37-year-old woman were taken to two Brooklyn hospitals where they were pronounced dead, police said.
Authorities were withholding the victims' names until family members had been properly notified.
Fire Department spokesman Jim Long said emergency workers responded just before 11 pm Saturday to a 911 call from a person stabbed at the residence in the Sunset Park neighborhood.
Adjoining two-story brick buildings sit on a tree-lined block in the working-class neighborhood where the stabbings occurred.
A neighbor May Chan told the Daily News it was "heartbreaking" to learn of the deaths.
"I always see (the kids) running around here," Chan said. "They run around by my garage playing. They run up and down screaming."