A 14-year-old boy killed his former girlfriend and her sister by starting a fire at their home in revenge for their break up, the Old Bailey has heard.
Maleha Masud, 15, and her sister Nabiha, 21, died at their home in Lessingham Avenue, Tooting, last June.
The jury heard the boy, who cannot be named because of his age, had searched Google for "how to burn someone's house down" the day before the fire.
Shihabuddin Choudhury, 20, Rasal Khan, 18, and the boy deny murder.
Mr Khan, of Earl Howe Street, Leicester; Mr Choudhury, of Coventry Road, Nottingham; and the boy also deny the attempted murder of Maleha and Nabiha's 55-year-old mother Rubina and their two brothers, Junaid, 18, and 24-year-old Zain.
The court was told the boy and Maleha had been in a "relationship of sorts" which was not serious.
Prosecutor Jonathan Laidlaw QC said Maleha had initially thought he was "nice and caring" but after they split she had complained he was "blackmailing her" about telling her mother about the relationship.
Mr Laidlaw said: "The two of them broke up and it was then that (the boy) threatened Maleha that if she did not continue in the relationship he would do something to her and her family.
"Why he should arrive at the extraordinary decision to burn down their house is really impossible to understand.
"It was obviously not the reaction of an ordinary and normal 14-year-old, however hurt he might feel about losing a girlfriend."
Mr Laidlaw said Mr Khan and Mr Choudhury had become involved to help the teenager get revenge on Maleha and her family.
The fire was started by petrol being poured through the letterbox of the Masud family home and then set alight while they slept, the court heard.
Died in hospital
Maleha suffered carbon monoxide poisoning and brain damage in the fire and died three days later.
Nabiha, who had recently become engaged, was also poisoned and suffered burns and died a month later in hospital.
The other family members managed to escape.
Mr Laidlaw said police searched the boy's house and found a petrol can with the same kind of fuel found at the Masud family home.
Experts examined his computer and found he had searched Google on how to burn down a house.
The trial continues.