It happened around 12:30 a.m. on Lockwood near the North Beltway.
Deputies said the victim's life may have been saved by the first deputies on scene.
"It was a serious stab wound. When deputies arrived, they applied a tourniquet using life-saving measures, most likely saving this 18-year-old's life," said a deputy.
The brother who is allegedly responsible for the stabbing was taken into custody.