MILTON, Mass. -- Relatives and friends of two young sisters slain Sunday in suburban Boston, allegedly at the hands of their brother, are reeling as they try to understand the grisly crime.

Grief counselors were scheduled to be available to students at Milton High School, where Samantha Revelus, 17, known as "Princesse" to her friends, attended classes.

Revelus and her 5-year-old sister, Bianca, were killed in their home, one of them as a police officer looked on in horror, authorities said.

A third sister, Sarafina, 9, narrowly escaped being killed when officers shot the girls' brother, Kerby, 23. The girl remained hospitalized Monday with stab wounds to her hands, abdomen and one of her legs.

"In policing, we see the raw human emotion every day, but to think that a human being could inflict such an atrocious, violent act on his own family is unbelievable," Milton Police Chief Richard G. Wells Jr. said Sunday.

Samantha was remembered by Milton High officials as a student who had many friends.

"Wonderful young lady. She was a great student. Just a sweetheart of a girl," Principal John Drottar said.

Police said as Samantha lay dying, she helped her younger sister call 911 just before 5 p.m.

"The assailant had the 5-year-old in his arms and was holding her hostage," Wells said.

The officer said he saw Kerby slash the girl's throat and then go after the 9-year-old. Investigators believe the rampage was provoked by a dispute with a neighbor Friday night that led to a fistfight.

Neighbors said the family has lived in the affluent neighborhood south of Boston that is also home to Gov. Deval Patrick for 20 years.

Investigators said the girls were attacked with a kitchen knife while their grandmother was in the basement doing laundry. Their parents were not home at the time. The mother is a nurse at a Boston hospital, Wells said.

Kerby was also a graduate of Milton High School but had a criminal record and had spent time in prison for gun possession. He had only recently been released from jail, authorities said.

Police said they were called to the house in 2004 on a report that Kerby had punched a woman living there.