“If you don’t get here fast, I’m going to kill this guy!”

A distraught Queens teenager made that warning to a 911 operator moments before allegedly stabbing her brother to death in a clash over a cellphone, sources told The Post yesterday.

Neighbors could hear screams from inside the first-floor Woodhaven apartment during a fight between Yocairis Diaz, 18, and Frank Fortuna, 20, at about 5:25 p.m. Friday.

Fortuna picked up a dog chain and Diaz pulled a knife, a law-enforcement source said.

The screams from the fight were soon replaced by cries of horror.

“Help me, help me! He’s dying!” yelled their brother, Kelvin Diaz, 19, according to a neighbor who rushed to their aid.

“Frankie was lying on the ground bleeding out, and his eyes were rolling back in his head,” the neighbor said.

Yocairis called 911 again to report the stabbing, dialed a second number, then fled with a neighbor in tow.

“I did it! I stabbed my brother,” she allegedly told the neighbor, who saw her a block from the murder scene.

“You should have seen how he had me,” Yocairis allegedly added, making a strangling gesture. “I didn’t know what to do.”

A black sedan with two women pulled up to the corner. Yocairis got inside and the car peeled out.

She was arrested at about noon yesterday and hauled into Queens Criminal Court, where she was awaiting arraignment on a charge of second-degree murder, said District Attorney Richard Brown said.

Fortuna was dead on arrival at Jamaica Hospital, police said.

Neighbors said the siblings often fought loudly and violently in the apartment, which they shared with Kelvin and their mother, who was absent.

“When they fought, you could hear the walls shake and furniture being slammed around,” said Bryant Aracena, who lives upstairs from the victim. “I was forced to call the police a few times.”

Yocairis was often “very hostile and a big troublemaker,” neighbors added.

“The mom would try to keep her out of the house because she would always fight with her brothers when she was there,” Aracena said. “The brothers would fight among themselves, too.

“It got to the point where the brothers came out into the street and got into a few fistfights,” he added. “It’s like they were trying to kill each other. These people have some serious problems.”

Their grandmother stopped by the house and said, “I don’t know what happened. I’m sad and angry, but I want answers. I want to know what really happened in that house. I hope the investigation will lead to the truth.”

Neighbors described Fortuna as a normal guy who liked to play basketball with his neighborhood friends and had been trying to get into boxing.

“He liked sports and video games,” said Patrick Bolden, 17.

“He didn’t deserve what happened to him — nobody does.”

Another neighbor, Ramon McPherson, added, “Despite the arguments, he loved his family and had respect for others.”

“He was a no-nonsense character, very serious guy,” sais McPherson 30.

“I don’t know how to explain what happened. It’s sibling rivalry gone wrong.”

Neighbor Kristina Anjarwati, 37, said, “You hear arguing all the time in this area. Nobody seems to get along. We are surrounded by it.”

Additional reporting by Jessica Simeone