The properties were set ablaze by the group amid rumours a Christian man had been having an affair with a Muslim woman.
The man's mother was stripped naked and paraded through the streets of her village, just south of the capital Cairo.
It took police two hours to arrive at the scene and they had ignored warnings of a possible attack, it is claimed.
General Bishop of the Coptic Orthodox Church in the United Kingdom, Bishop Angaelos, told Premier's News Hour the police response time was "utterly unacceptable".
Now Egypt's president has condemned the attack and promised justice for the 70-year-old woman.
"We are all one and the law must take its course," Abdel-Fattah el-Sissi said.
Christians in Egypt regularly come under attack. Earlier this month, a campaigner defending the rights of Coptic Christians was arrested.