A 14-year-old schoolgirl stabbed her 16-year-old sister in the back with a carving knife after a row about a boyfriend, a court has heard.
The teenager, who cannot be named for legal reasons, pleaded guilty on Tuesday to manslaughter at Bradford Crown Court. She denies murder.
Her sister was found with serious injuries at their home in Halifax, West Yorks, in March and died in hospital.
The mother told the court the girls started arguing before the stabbing.
The court heard that the defendant told her sister that her boyfriend "wouldn't amount to much, only working in Morrisons".
At this point, the argument escalated into a physical fight, with the older sister grabbing the younger girl by the hair.
The defendant then went into the kitchen and ran back into the living room with a 20cm carving knife.
She ran past her mother and shouted at her sister "you're going to get it".
The mother told the court: "They were struggling. I couldn't see where the knife was. I just saw blood spraying up on the wall."
She said she told her daughter to put the knife down but the girl said: "I don't care, I'm going to have you".
Afterwards, the defendant dropped the knife on the floor and the wounded sister grabbed it and stabbed her in the back of the leg.
The mother said the defendant "started screaming and crying 'I'm sorry, I love you'".
The older sister then walked into the hallway and complained that she couldn't breathe or see.
She was taken to hospital in an ambulance but died hours later.
The mother told the court her daughters had "got on really well up to about a year ago and then they started fighting and arguing."
She said the girls argued about many things, including chores, friends, boyfriends and "nicking" clothes.
The older girl used to taunt the defendant about being fat, and also about turning out like their eldest sister, now aged 18.
The 18-year-old went into care when she was 13 after becoming uncontrollable and pouring a kettle of boiling water over her mother's head, the court heard.
Tom Bayliss, prosecuting, said: "The prosecution don't suggest that this was anything other than an impulsive act, carried out by a young woman who had lost her temper.
"The prosecution do say that this was a deliberate act and that she did at least intend to cause really serious bodily harm."
The trial continues.