Police say 43-year-old Yvette Randolph splashed gasoline inside the brownstone on Chauncey Street in the Bedford-Stuyvesant section just after 9 a.m. and then set it ablaze.
Sources tell Eyewitness News that her target was an ex-boyfriend who doesn't even live in the building anymore.
More than 100 firefighters responded to battle the intense flames.
Twelve people were injured, including children, and five people were rushed to area hospitals with critical injuries.
Firefighters also made several dramatic rescues.
Thankfully, all the victims are expected to survive.
The brownstone has reportedly been in the same family for three generations, currently owned by Rhonda Scott.
Among the injured were Scott's niece and two nephews, 12-year-old Zahir Storey, his 4-year-old cousin Alston Storey and Alston's 13-year-old older sister, Keara Storey. They remain in a Brooklyn Hospital.
"I don't know what to say, and I don't know what to do," Scott said. "This is in God's hands...They took in so much smoke, so much soot, so they have to get it out of their lungs. Not good to see no babies like that."
Randolph is charged with second-degree arson.