The Harris County Sheriff's Office said they believe the man's 17-year-old relative, possibly his brother, was the one who stabbed him.
Investigators said the stabbing happened at the Chisholm Trail Apartments, but it's unclear what led to the incident.
First responders drove the 18-year-old to the hospital for his stab wounds and said he was in critical condition.
HCSO Det. David Crain said the 17-year-old was detained but was not named a suspect during a press conference.
Crain said investigators are looking into who was involved in the confrontation and what led to it.
"We're not 100% sure on their relationship. We've been told they may be brothers," Crain said.