Joseph Bravio Mendez of 1036 Leonard Road is being held without bond on first-degree murder and assault with intent to kill charges.
He had his first court appearance Wednesday and asked for a court appointed attorney. His next court appearance is scheduled for February 2.
Investigators say the teen was on the phone with his ex-girlfriend when he became enraged and attacked his sister Juillian Mathews with a kitchen knife. The girl was stabbed in the neck and died later at UNC Hospitals.
Mendez's mother - 41-year-old Lydia Etanislao - tried to intervene and was wounded in the scuffle. She was listed in critical condition Tuesday afternoon.
After the alleged attack, Mendez ran to his neighbor's house and asked them to call for help - telling them that he did something terribly wrong.
The teenage suspect was found at the neighbor's home, lying on the floor in tears, saying "...all I know is I hurt my mom and don't know why."
He also told investigators that he dropped the knife in the yard. It was located near the porch.
Officials said Mendez was a student at Louisburg High School. Calls to the Franklin County school system were not immediately returned.