One of the gangbanger brothers involved in the slaying of a heroic Brooklyn mom gave the chilling go-ahead to take out a member of a rival crew, authorities said yesterday.

“Go get your gun and do what you have to do,” Jonathan Carrasquillo told his brother, Andrew Lopez, moments before they fired from a Brownsville rooftop, according to prosecutor Terry Sand.

The bullets injured Cheanne McKnight, 11, and a 31-year-old woman — and fatally wounded Zurana Horton, 34, who was trying to shield kids from the fusillade Friday afternoon, authorities said.

Horton was a mother of 13.

Carrasquillo, 22, and Lopez, 18, were remanded at their Brooklyn Supreme Court arraignments yesterday.