An unhinged 45-year-old went wild during a clash with his estranged in-laws on Oct. 1, killing his former sister-in-law and wounding three other relatives — including a four-year-old boy — during a bloody rampage inside the family’s Sheepshead Bay home.

Police allege Han Min Pan forced his way into the apartment between Gravesend Neck Road and Avenue W at 5:45 pm, looking for his ex-wife.

Realizing she was not there, Han demanded money from his former in-laws — who were just sitting down to eat dinner — but they refused.

Outraged, Han grabbed a kitchen knife and lunged, stabbing 29-year-old Jei Mei, his ex-wife’s younger sister, repeatedly. He also lashed out at his ex-wife’s 38-year-old sister Joyce, his ex-wife’s 62-year-old mother and Joyce’s 4-year-old son.

Mei died of her injuries after being rushed to Coney Island Hospital. Han’s other victims were all hospitalized with non-life-threatening injuries. Joyce and her mother were both stabbed in the stomach and the chest, police said. The boy had been stabbed in the shoulder.

It was unclear by late Monday just how long Han and his wife had been separated. Neighbors were shocked by the bloodshed, claiming E. 18th Street has always been a quiet block.

Police said Han, who lives in Bensonhurst, ran off as terrified witnesses called 911. Investigators nabbed him a few hours later, charging him with murder, attempted murder, burglary and criminal possession of a weapon.

Mei’s death is the fourth murder to be investigated in the 61st Precinct this year.