Brittany Cornett said her sister had separated from Stidham in October after a three-year relationship.
College President Stephen Greiner said about 30 students were on campus at the time of the shooting. He said college staff responded quickly by securing the school's buildings.
"Our thoughts and sympathy are with the families of the victims of this tragedy," Greiner said.
Caitlin Cornett was a student at the college during the 2010-11 school year and had recently signed up for a February course.