The bodies of Sarah and Amina Said were found Tuesday in their father's taxi. Police were seeking Yaser Abdel Said on a capital murder charge, The Fort Worth Star-Telegram reported.
Students at Lewisville High School held up posters of the two sisters as their mother and brother spoke. The vigil was organized by students.
"They will be missed," their weeping mother said. "They were always happy, even when I was sad."
Their brother, Islam, defended the family's religion while blaming his father's alleged act.
"Religion has nothing to do with this, and it was very wrong. Islam is not a bad religion," he told the crowd.
A passerby found the bodies Tuesday evening in a hotel parking lot. About an hour earlier, one of the sisters apparently called 911 on a cell phone but was unable to say where she was.
Friends described Amina, 18, and Sarah, 17, as pretty and popular girls who did well in school. The family were immigrants from Egypt.