Joshua David McEachern, 17, was ordered held without bond on a murder charge after a brief court appearance. It was not clear if he had an attorney.
Sheriff's deputies did not release details of the dispute that led to the shootings around 9:30 p.m. Sunday at the McEachern home in Greenville. They said 59-year-old Kathy McEachern was killed and 21-year-old Daniel McEachern was in critical condition. More charges were expected later Monday.
After the shootings, deputies said, Joshua McEachern called 911 and cooperated with authorities when they arrived but didn't tell them why he shot his relatives.
Master Deputy Sam Cureton said authorities were working to find out where he got the gun. Cureton said authorities had no records of domestic disturbance calls at the family's home, and McEachern had no previous arrest record as an adult or juvenile.
The teen's grandmother told The Associated Press her grandson was a sweet boy but never seemed normal.
"I don't know how to really say this, but he was a child that was not 100 percent, mentally," Fran McEachern said from her home in Tifton, Ga.
She said her son and Kathy McEachern battled in court over custody of Daniel and Joshua after divorcing a decade ago. Their father saw his sons every other weekend, but their mother had primary custody.
As for her younger grandson, whom she last saw more than a year ago when the boys came to visit and fish in a pond near her home, Fran McEachern said she did not know if he had been on any drugs or medication.
"He was not normal to the degree of what he should be," she said. "I really don't know what created it. And somehow I don't much want to know."
No one answered the door at the split-level, red brick family home Monday afternoon, but drops of blood were smeared on the front door frame and on the stairs leading down from the front porch.
All of the windows were covered, some with blinds and some with sheets and blankets. But four long-stemmed red roses, still fresh, could be seen on the sill of a small window over the kitchen sink. Kendra Garcia, who lives across the street, said she'd noticed both boys were often rude to their mother but did not expect the disputes to turn violent.
"I knew they were rude kids, but they seemed normal," the 23-year-old stay-at-home mother said. "We would never think he would hurt his mother."
Meg Kinnard reported from Columbia and can be reached at http://twitter.com/MegKinnardAP