A reserve deputy with the Harris County Sheriff's Office shot his brother-in-law during an argument, police said.
The shooting happened shortly before 7 p.m. on Goforth and Balkin Street Tuesday.
Investigators said the reserve deputy and his wife were arguing about a motorcycle and the wife called her brother.
"It started with a domestic disturbance," said Lt. John Zitzmann with the Houston Police Department. "His wife insisted that he leave. He wouldn't so she called her brother and said, 'Come over here. My husband is giving me some trouble.'"
When the brother arrived, he got into an argument with the reserve deputy and slapped him in the face.
"He slapped the reserve officer in the face, and I guess it was just an instinctive reaction to go for his weapon and defend himself. The reserve officer discharged his weapon one time, striking his brother-in-law in the arm," said Zitzmann.
The brother-in-law was shot in the arm and transported to a hospital with non-life-threatening injuries, authorities said.
The reserve deputy was arrested and charged with aggravated assault.
"He had to give up his ID and his badge," Zitzmann said.
According to HPD, he has been serving as a reserve deputy for a couple of months. He was previously a civilian jailer for Harris County.
His name was not released.