A 10-year-old boy shot his sister and then turned the gun on himself, investigators said.
Walker County sheriff's deputies said the shooting happened on Duke Road near Highway 30.
Detectives said the boy walked into his sister's room and opened fire.
The 15-year-old girl was flown to Memorial Hermann Hospital in critical condition. Her name has not been released, but officials said she is a student at Huntsville High School.
Detectives said the boy fatally shot himself. His name has not been released, but officials said he was a student at Huntsville Intermediate.
Investigators have not said how the boy got the gun. The motive for the shooting is under investigation.
Deputies said the children's mother was at home at the time of the shooting but was apparently unaware that her son had a gun. The children's father was at work at the time of the incident, investigators said.
Officials have not said if the parents will face any charges.