Thaddeus Boone of Newark is a boyfriend of an 18-year old resident of the apartment. But she was not there at the time he allegedly set the fire, authorities say.
Detectives believe he set the fire as part of an ongoing dispute with her.
Boone is facing multiple charges of attempted murder, asssault and arson, along with a burglary charge.
Two of the 18-year old's siblings, an 8-year old girl and a 6-month old boy, were saved by two firefighters responding to the blaze.
For one of the firefighters, it was his very first fire call.
The flames broke at 2 a.m. while the children were sleeping at the Wyckoff Gardens housing development on Third Avenue in the Boerum Hill section.
Firefighters say the flames were contained to an apartment on the fifth floor of the 21-story building. Jordan Sullivan, who was on his first fire call, put everything he learned to the test.
"It wasn't visible in there, so I was feeling for him," says Sullivan.
Sullivan crawled on the ground through the blinding smoke, and found the baby boy in the bedroom.
He first put out the flames so he could enter the bedroom, and so a second firefighter could enter another bedroom where the 8-year-old was trapped.
"It's an unbelievable experience ? it's an honor to be able to serve the community," says Sullivan, "I still have a lot to learn."
The two children remain in critical condition at the Weill Cornell burn center.
The 8 year old girl attended PS 38 in Brooklyn.
Her school is now planning a fund raiser for the family.
The hero firefighters have been invited and the childrens' grandmother is expected to thank them personally.