Collingdale police are trying to piece together the details of a twisted family dispute, in which a teenage boy was stabbed in the chest after his mother said she caught him sexually assaulting his 7-year-old half-sister.

When police and paramedics responded to a house in the 200 block of Pusey Avenue Sunday evening, they were confronted with a bizarre crime scene: A woman was lying on her back near the kitchen, calling out for help, and her 13-year-old son was bleeding from his chest with a knife in his hand.

"We're still interviewing and trying to piece together exactly what happened," John Hewlings, police chief in the Delaware County municipality, said yesterday.

The 28-year-old mother, whose name is being withheld by the Daily News to protect the family's identity, was arraigned yesterday on charges of simple and aggravated assault, recklessly endangering another person and possessing instruments of crime.

She failed to post bail and was taken to Delaware County prison.

The woman told police she had passed her daughter's bedroom Sunday and noticed the room was empty. She then opened her son's bedroom door and found him in bed with the girl, engaging in sex acts, according to the criminal complaint.

"The mother came across them in a compromising situation," said Sgt. Robert Adams, who responded to the scene.

The woman told her son she was going to call his father, according to the complaint, which sparked a fight between the two.

The altercation proceeded downstairs, where they both reached for a kitchen knife, Adams said.

"She grabbed the handle, he grabbed the blade, then they struggled," he said.

During the fight, the son was stabbed in the right side of his chest, below his rib cage. Police said there was a pool of blood on the floor when they arrived.

The son was treated by paramedics and taken to Children's Hospital of Philadelphia. Hewlings said that the injuries were not life-threatening and that the youth was expected to be released yesterday.

The daughter was interviewed yesterday by Delaware County Children and Youth Services, which investigates allegations of child sexual abuse.

As of late yesterday afternoon, the son had not been charged with any crime. *