Seven members of the Syrian Army also died as a result of fighting with alleged army defectors, the activist group said.
The Syrian Observatory and the Local Coordinating Committees, an opposition group that organizes protests, said three sisters from one family were killed in Qoseir, near Homs, when security forces and the army shelled homes in response to defections.
The three sisters, from the Zoghby family, were aged 17, 15 and 11, the groups said.
We cannot independently confirm events in Syria, which restricts international journalists from accessing many parts of the country.
More than 20 people died Monday in Homs during intense clashes between security forces and defectors from the military, activists reported.
The United Nations estimates that about 3,000 people have died in Syria in the seven months since the government's crackdown on protesters demanding democratic reforms began.
The government says it has been making efforts to respond to citizens' grievances about the country's political system, and blames armed groups for stoking the violence.
