The incident took place around 6 p.m. as a blaze erupted at the two-story Salisbury residential building, according to the Office of the State Fire Marshal.
At least eight units were in the wood-frame apartment building, according to a report.
When the 11-year-old boy discovered the fire, he initially left the building before realizing that his sister was still inside, the report noted.
Recognizing his sister was in danger on the second floor, the boy ran back inside and returned with the young girl.
The boy suffered minor injures, including a slightly burned arm, authorities said.
“His injuries were so minor they did not require on-scene medical attention,” the state fire marshal said.
Firefighters managed to put out the blaze in under 10 minutes.
At least $250,000 in damage was done to the building’s structure, and an additional $40,000 in damage was caused to the building’s contents.
Two of the apartment units were rendered uninhabitable, but all the buildings occupants inside at the time of the blaze are safe, authorities said.
The cause of the fire has been identified as an electrical event that took place inside a second-story bedroom, but it has not been made clear where the parents of the children were during the fire.