A 17-year-old boy repeatedly knifed by a rampaging in-law who murdered the boy’s mother and then set their Queens home on fire was saved by a pregnant off-duty Port Authority police officer who heard screams for help and raced over, the Daily News has learned.

The Port Authority cop lives in Flushing and was on her way home around 3:30 p.m. Thursday when she heard someone’s panicked cries and saw the teen bleeding in the street on Smart St. near 45th Ave.

The teen and his 50-year-old mother, Qiujin Shi, had just been attacked by Shi’s 27-year-old son-in-law, according to police.

The cries for help came from Shi’s adult daughter, who is married to the son-in-law and was standing outside the home as the carnage unfolded, the cop said.

“When I got there, the wife of the stabber was sc

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