Osvaldo Rivera, 31, is suspected of breaking into the children's home around 2 a.m. Sunday and attacking the 12-year-old girl, reported the Camden Courier-Post.
County prosecutor Warren Faulk said little Dominick Andujar saved his sister's life.
"He came to her assistance while she was being assaulted, and as a result of that she was able to get out of the house," Faulk is quoted as saying.
The girl's throat was also cut, but she managed to flee to a neighbour's house for help, according to the report. She underwent surgery for her wounds. Two other girls in the home, aged 9 and 14, were unharmed.
Police allege Rivera was high on "wet," a combination of marijuana and PCP, at the time of the attack.
Rivera is charged with murder and faces additional charges related to the alleged sexual assault. He is being held on $5 million bail.