A late-night livery car ride through Flatlands, Brooklyn, turned tragic for a brother and sister this weekend, when a gunman trying to rob the vehicle fired a bullet into the brother’s head.

The gunman had already shot up a house party and pistol whipped a club’s bouncers early Saturday before approaching a livery cab stopped at the corner of Utica Avenue and Avenue O, said Brooklyn Councilman Jumaane Williams.

A struggle then ensued in which the gunman pointed his weapon at the livery car driver, but instead ended up hitting 23-year-old passenger Nikita Grebelskiy, according to both police and Williams.

Grebelskiy has since been declared brain dead.

His sister, Elizabeth Filatova, 25, was reportedly sitting next to him at the time of the incident.

Grebelskiy’s mother is keeping vigil at Kings County Hospital, according to the New York Daily News.

“It’s the most terrible thing in the world that can happen,” she tearfully told the News. “His brain is dead. According to the state of New York, he’s dead.”

“I cannot imagine the horror a mother must feel knowing her child is brain dead because of the senseless act of one man and his gun,” Williams said in a statement.

At the earlier house party on E. 54th Street, a 35-year-old man was shot in both ankles, a 34-year-old woman was shot in the stomach and left leg, a 32-year-old woman was grazed in the right thigh and a 44-year-old man was grazed in the left knee, cops said.

Police were not yet explicitly linking these incidents as of Sunday afternoon. But Williams said that one man, identified in news reports as Michael Magnan, 19, had been arrested in connection with all of the bloodshed.

Timeline of a bloody rampage

1. At about 12:50 a.m. Saturday, the gunman shot up a house party on E. 54th Street. Four people were hit, including a woman who remains in the hospital in stable condition.

2. The gunman next reportedly pistol whipped two bouncers at the eSavoy Lounge on Flatbush Avenue.

3. At around 2:30 a.m., the gunman shot livery cab passenger Nikita Grebelskiy in the head at the corner of Utica Avenue and Avenue O.

4. The gunman then fled into the nearby club 9jaVilla, where he threatened to shoot another bouncer.

5. The bouncer subsequently notified a passing police car, and the gunman was apprehended soon thereafter.

A cab driver’s remorse

Cab driver Eric Martine of Gateway Car Service told the Daily News that the gunman went through his pockets before demanding Grebelskiy’s and Filatova’s cash and jewelry.

“That’s when I punched him and hit the gas,” Martine said. “He (shot) out the back window, and shot the kid in the head.”

“I should have took that gun from him,” Martine added to the News. “I should have thought faster.”