Dante Daniels caught his mother's ex-boyfriend Deandre Chaney Jr.
After the attack, Dante was declared brain dead, and died six days later.
The suspect is said to have also used a hammer and a knife on Danae, as well as on the children's 27-year-old mother, Elizabeth Salone.
23, attacked Dante Daniels with a hammer and lighter fluid, Fox 40 reported, citing a criminal complaint.
"Trying to save his sister from this child molester, and that's why he was beat the worst", said Dante's grandmother, Monique Brown.
After his death, the 8-year-old's heart was donated to a 4-year-old, his grandmother says.
Investigators say Chaney, who has a lengthy rap sheet including rape, robbery and assault charges, committed "lewd acts" on the girl on September 1 before beating her, her brother and the children's mother, 28-year-old Elizabeth Salone, with a hammer.
A 8-year-old kid in California was executed attempting to shield his more youthful sister from their mom's ex.
"Haha. He was a little", she said. Wylie saved his sister but he was hit by the auto himself and died a day later succumbing to his injuries in a hospital in Charlotte, North Carolina.
Upon his capture the following day, Chaney Jr was charged with murder and attempted murder charges, and now faces a charge for lewd acts with a child under 14.
The Bee, quoting an arrest warrant, said Chaney cut off "his parolee ankle monitor and [hopped] on an Amtrak train bound for Salt Lake City".
Although Dante's mother and sister both survived the attack, Brown says Elizabeth will probably never see out of her left eye again and her granddaughter Danae will need a lot of help. He also pleaded no contest to assault charges in 2011.
In response, a GoFundMe page has been set up to help the family pay for medical bills and recovery.