Police in Texas are crediting a 12-year-old boy with preventing the possible murder of his great-grandparents after his half brother allegedly attacked them with a knife on Sunday.

The brothers were in a car with their great-grandparents, ages 76 and 92, in Katy when an argument broke out, the Harris County Sheriff’s Office said in a press release.

Lucian Johnston, 20, was upset that his great-grandparents said he could no longer live with them because of his behavior, police said.

He allegedly then began stabbing them in the head and neck with a pocketknife, KTRK reported.

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His younger brother, Sacha, who was also in the back, intervened. Police said he managed to wrestle the knife away from Lucian and throw it out the window, successfully stopping a potential double murder.

“The real hero in this ordeal is a 12-year-old that was also in the backseat at the time of the brutal attack,” a sheriff’s deputy said.

Lucian reportedly gave up on the fight and fled the scene. He was arrested Monday morning and charged with aggravated assault with a deadly weapon, police said.

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The great-grandparents drove themselves to the hospital where they were treated before being released Monday night. Both are expected to make a full recovery.