The extremists broke into the family home in the village of Al-Mazira, located in the central province of Hama, late Sunday and shot some of the victims and stabbed others.
The family belongs to the Ismaili sect, part of the Shia branch of Islam.
Meanwhile, in the northeastern province of Deir ez-Zor, at least 13 people were killed, including civilians, in heavy bombing and fighting between the IS and tribal militiamen to gain control over the town of Sowaidan Jazira.
The town was finally taken by the IS, formerly known as the Islamic State of Iraq and Greater Syria (ISIS), which had retreated Friday after clashes with one of the tribes in the area.
More than 171,000 people have been killed since the conflict began in Syria in March 2011, according to the latest reports by the UN.