After playing in a nearby park with their brother and a friend on the evening of April 5, 2008, 14-year-old Acia Johnson took her 2-year-old sister, Sophia, back to their West 6th Street home and carried the little girl up to her third-floor bedroom, a Suffolk County homicide prosecutor told a Superior Court jury this morning as their alleged killer’s trial began.

“What Acia didn’t realize,” Assistant District Attorney David Fredette said, “was that with every step she took, she was sealing her fate and Sophia’s fate.”

As the children slept, Fredette told jurors and a rapt courtroom audience, NICOLE CHUMINSKI (D.O.B. 5/19/82) arrived at their home and began to bang on the door.

“She had been humiliated at a wedding hours earlier by Acia’s and Sophia’s mother,” Fredette said.

When no one answered the door, the prosecutor said, Chuminski “willfully and maliciously” set fire to their home.

“The conditions were perfect for that fire to spread, trapping Acia and Sophia on the third floor,” Fredette said.

The teenaged victim “picked up the baby and brought her to the closet,” the prosecutor continued. “She cradled that baby. And that’s the way the fire department found them.”

Chuminski is charged with two counts of first-degree murder for the girls’ deaths. She is also charged with two counts of assault and battery with a dangerous weapon – smoke and flames – for injuries sustained by the girls’ mother and brother, as well as a single count of arson.

In the hours and days after the conflagration, Fredette said, investigators from the Boston Fire Department and Boston Police Department determined that the fire had been deliberately set.

“Accelerant-sniffing dogs came to the scene and hit on something,” Fredette said. Evidence from the scene was brought to the Massachusetts State Police Crime Laboratory, where chemists found the presence of acetone – a chemical found in many solvents.

Chuminski’s clothing was seized, the prosecutor said, and also tested positive for acetone.

Fredette said jurors would hear about Chuminski’s “volatile, emotional relationship” with her girlfriend, the children’s mother, and that she was “seething” about the incident at the wedding.

Chuminski returned to Boston at about 2:00 a.m., Fredette said, and the fire was set at about 3:00 a.m. A few minutes past 3:00, Chuminski allegedly made arrangements to obtain drugs, telling a witness about the fight and saying she threw a bottle through the window of the West 6th Street home.

In a subsequent interview with Boston Police homicide detectives, Fredette said, Chuminski later contradicted those statements, denying she had been near the house.

Fredette told jurors they would hear that statement and hear testimony from multiple witnesses implicating Chuminski.

“You will have all the tools you need at the end of this trial to find the defendant guilty,” he said. “Nicole Chuminski went to that house that morning and intentionally set that fire.”

Catherine Yuan is the DA’s victim-witness advocate assigned to the case. Chuminski is represented by attorney William White. Judge Frank Gaziano is presiding over the case, which is expected to last about two weeks.