"My sister Asmaa was martyred," Ammar al-Beltagi said via Twitter on Wednesday.
The 17-year-old girl was shot in the chest and back when security forces violently dispersed a six-week-old sit-in by supporters of ousted President Mohamed Morsi in Rabaa al-Adawiya Square in eastern Cairo.
According to medics and eyewitnesses, at least 163 people were killed and thousands injured during the dispersal of the two main pro-Morsi protest sites at Rabaa al-Adawiya and Giza's Nahda Square.
The Egyptian Ambulance Organization, however, has put the death toll in the two squares at 13.
The Interior Ministry, meanwhile, said three police officers had been killed in the violence.