A teenage boy who attempted to stab his ex-girlfriend to death at the behest of a “jealous” friend, has been sentenced to 16 years in prison.

Rhett Carty-Shaw, 17, launched the attack on Iman Nassir, also 17, at her home in Openshaw, Greater Manchester, in May.

He was jailed for attempted murder and possession of an offensive weapon at Manchester crown court on Friday, alongside Kyro Mori Akhiro, 17, who was also sentenced to 16 years for encouraging and assisting a person to commit murder.

The pair plotted to kill Nassir after Akhiro, formerly known as Sarah Mohamed, discovered the defendant was still seeing the victim.

Although Akhiro had ended the relationship with Carty-Shaw, she egged him on to “prove” that he loved her. During the attack, she texted him asking him to film it and bought Dettol wipes for him to clean the knife used.

Judge Alan Conrad QC, sentencing, told the defendants: “You are both selfish narcissists who believe that the world revolves around you.”

Carty-Shaw had been acquitted by a jury of raping Nassir before he stabbed her, and it had been established they had consensual sex. However, Conrad added that it was “disturbing” that he was “prepared to have sex with a young woman he was at least contemplating killing. What sort of person behaves in that way?” The pair also had a baby boy who was two months old at the time of the attack.

Katherine Pierpoint, defending Carty-Shaw, told the court that her client’s intellect had been deemed so low by a psychiatrist that he had narrowly missed the criteria for an intellectual disability, and was particularly “susceptible to manipulation”.

However, she added that this did not affect his ability to know the difference between “right and wrong”.

During the trial, the court heard that after a meeting between the defendants the day before the attack, Carty-Shaw made internet searches including one asking “how long do people get sentenced for murder?”.

The next day, he turned up at Nassir’s home with a change of clothes and a large kitchen knife in his bag, while Akhiro was seen loitering close to the address.

After the attack, Carty-Shaw sent a text to his co-defendant reading: “I did it to prove I love you … There was no other way to keep you.”

Nassir, along with her son, mother and siblings, has since moved away from Greater Manchester in order to “feel safe”. She received stab wounds to her face, head and neck after Carty-Shaw rained blows on her from above and spent four days in hospital, undergoing several surgeries.

In a victim impact statement, Nassir told the court she was undergoing therapy because her son served as a reminder of Carty-Shaw, while her siblings were also receiving mental health support after witnessing her bleeding heavily before the ambulance arrived. She also said she finds it difficult to carry her baby because of the injuries she sustained.

Speaking after the sentencing, Nassir, who has waived her right to anonymity, said it had been “hard to move on from the situation”, but “it put my mind at ease to know that they’re in prison now”.

She added that she believed Akhiro was “manipulative and delusional” and had turned Carty-Shaw into a “Jekyll and Hyde” figure.

Although the pair had had a two-and-a-half-year relationship after meeting at the Manchester Academy school in Moss Side, Carty-Shaw began seeing Akhiro around the time Nassir became pregnant.

“She didn’t want to accept the fact that I was there before her,” Nassir said. “She was living her own fairytale, stuck in her head.”