A jealous 14-year-old torched the house of his former girlfriend, killing her and her sister, after she split up with him less than 48 hours earlier, the Old Bailey heard today.

The boy used the internet to search “how to burn someone's house” 16 hours before the attack on the Tooting home, the court was told, and saved a photo of the burnt out house after the arson as the screensaver on his computer.

Prosecuting, Jonathan Laidlaw QC told the court the relationship between the boy and Maleha Masud, which was non sexual, was the main reason for the murders and the boy had rang the family home and threatened to tell her mum of their relationship.

He said: “It is to [the boy] we look for the reason for this attack, although there was, and there never could be a good or sensible explanation for the terrible things that he and his co-defendants did.”

The boy was helped with the arson attack by two other men, who drove to London from the Midlands to help him “get revenge”, Mr Laidlaw said.

The boy and Maleha split last year on Friday, June 19 at about 7.30pm, Mr Laidlaw said.

“The two of them broke up and it was then that he threatened Maleha that if she did not continue he would do something to her and her family,” he told the court.

The boy, who is from Croydon but cannot be named, and two men, Rasal Khan, 19, from Leicester, and Shihabuddin Choudhury, 21, of Nottingham, deny two counts of murder and three counts of attempted murder.

On June 21 last year it is alleged the three helped poured petrol through the letterbox of the semi-detached house in Lessingham Avenue at about 4.30am.

A driver, who was only in the area because she was lost, alerted emergency services when she saw flames coming out of the house about 4.32am, Mr Laidlaw said.

The driver shouted “fire” and threw stones at the house to wake the family as flames swept through the property, the court heard.

The family’s youngest son, Junaid, 18, was found by firefighters in his bedroom after suffering burns and serious smoke inhalation damage.

He spent 45 days in intensive care following the attack.

Maleha, who shared a room with Junaid, died four days after the arson attack from burns and smoke inhalation.

Nabiha, a South Thames College student, who was 21 and engaged to be married, was resuscitated by paramedics at the scene but died a month after her sister from organ failure, brought-on by inhaling smoke.

Their brother Zain, 24, and the family’s mother Rubina, 55, both escaped the fire by jumping out of a first floor window.

Neighbours managed to break into the house through a back door but were beaten back by flames.

Mr Laidlaw said evidence from mobile phones, traffic cameras, CCTV automatic numberplate recognition software, placed all three defendants in the area at the time of the attack.

And,Mr Laidlaw said, when questioned by police after Maleha's death the boy said, "I can't believe she died".

The case continues.