Nicki Minaj's brother reportedly threatened to banish his stepson if the boy failed to keep quiet about the alleged rape of his sister, a court has heard.

Jelani Maraj is currently on trial in New York for sexually assaulting his pre-teen stepdaughter on a number of occasions in the months leading up to his December, 2015 arrest. He has denied all allegations.

On Monday (23Oct17), the girl's brother took the witness stand at Nassau County Supreme Court and detailed the night he reportedly walked in on Maraj having sex with his sister, who was 11 at the time.

The kid told the jury how he had gone to look for his older sibling after breaking his pencil, only to find her with her pants "around her ankles", and their stepdad standing over her with his underwear "close to his knees".

"I thought it wasn't right," the now-10-year-old testified. "It looked weird to me, so I ran upstairs."

Maraj later confronted the boy and is said to have repeatedly hit him in the face: "He asked me if I had seen anything," the child recalled. "I said, 'Yes.' He slapped me on the cheek."

The 38-year-old then allegedly threatened to ban the boy from seeing his mother, Maraj's wife Jacqueline Robinson, if he told others what he had apparently witnessed.

"Jelani said I wouldn't see her again if I told on him," the youngster claimed. "His face was kind of mad."

During the witness' cross-examination, Maraj's lawyer, David Schwartz, questioned what the boy had actually seen, noting that the lights in the bedroom his sister had been in had been switched off.

"You would agree that when the light is not on, the room is very dark?" he asked, prompting the boy to admit, "Yes."

Schwartz has argued the whole case is a set up, accusing Maraj's estranged wife of only marrying his client four months before his arrest as part of an extortion plot to win a $25 million payout from rap superstar Nicki.

Robinson filed for divorce last year (16).

Reports suggest the No Frauds hitmaker will be the defence's star witness, although it's not known when she is scheduled to take the stand.

She has never commented publicly on her brother's legal woes, but has stood by his side by putting up two of her homes for collateral to cover his $100,000 bail.

The trial continues.