A 43-year-old mom was stabbed to death outside her Staten Island building while defending her 12-year-old daughter during a large social media-fueled brawl this week, according to law enforcement sources.

Jennira Roundtree was knifed multiple times in the chest around 7:30 p.m. Tuesday outside her building on Henderson Avenue near Alaska Street, part of NYCHA’s West Brighton Houses, authorities said.

She was trying to come to her daughter’s aid when someone in the group turned on her and stabbed her to death, according to the sources.

The burst of deadly violence was social media-motivated, the sources said.

A 30-year-old neighbor told The Daily News that a group of girls picked a fight with Roundtree’s pre-teen daughter — prompting the mom to jump to her defense.

Roundtree’s friends had initially called the girl’s older sister for help, but the quick-acting mother got involved sooner, the neighbor said.

“Some girls come in to fight with the little girl and some friends, they go upstairs and then the sister was coming,” the neighbor said. “I don’t know where her mom came from. Maybe she was outside [already].”

The neighbor said she couldn’t fully see the mayhem because of scaffolding, but heard what happened. She ran downstairs and already saw officers taking the mortally wounded Roundtree away.

“The police took her,” she said. “When they carried her, I saw she had a lot of blood. I heard her say, ‘I can’t breathe!’ That’s the only thing she said.”

Roundtree was rushed to Richmond University Medical Center, where she was pronounced dead.

About 20 people were involved in the melee — mostly young teens joined by an adult believed to be in their mid-20s, cops and sources said.

Authorities confirmed that a daughter of the victim was on the scene, but was uncooperative.

A knife was used in the deadly attack, sources said.

Roundtree was found laying on top of an umbrella, indicating the shards may have also been used to slash her, according to the sources.

No arrests had been made by Thursday.

“This neighborhood was important to her because she wanted different for all the kids coming up,” one of Roundtree’s friends told WABC. “And she was trying to make a difference, and she was.”

A memorial set was set up outside the West Brighton Houses Thursday afternoon, with candles fashioned in the shape of a “J” for Jennira. 

Photos of Roundtree were taped to the building foyer windows.

But a young man who identified himself as Roundtree’s son declined comment to The Post and told a reporter to leave the NYCHA complex.