A FLORIDA WOMAN holding her 11-month-old son in a baby carrier was shot dead by her 14-year-old brother while trying to defuse a row over Christmas gifts he was having with a 15-year-old brother who was also armed, authorities have said.

The 15-year-old then shot the 14-year-old, though not fatally, for killing their sister on Sunday in Largo, Florida, the Pinellas County Sheriff’s Office said in a news release.

The argument started while the three siblings were Christmas shopping with their mother and the sister’s two sons, aged six and 11 months.

It continued when they went to their grandmother’s house where the sister, 23, told the younger brother to stop arguing with his older brother because it was Christmas Eve.

The younger brother then told his sister he was going to shoot her and her infant, and he then shot her in the chest, the sheriff’s office said.

The older brother then shot his younger brother outside the home because of what he had done to their sister and he fled the home, tossing his weapon in a nearby yard, authorities said.

He was taken to a mental health facility after he was found because he had threatened to harm himself.

Once he is released from the mental health facility, he will be taken to a juvenile detention centre, the news release said.

The 14-year-old brother was charged with first-degree murder, child abuse and possessing a firearm as a delinquent.

His 15-year-old brother was charged with attempted first-degree murder and tampering with evidence, the sheriff’s office said.