The unidentified girl appeared at a hearing at the Depke Juvenile Center in Vernon Hills, where she was charged with murder.
The girls lived with their mother in the same Mundelein home, about 30 miles northwest of Chicago, and attended the same Lake County school, investigators said at a news conference Wednesday.
A family friend described the siblings as close. Never, she said, was there any indication the 14-year-old was capable of harming anyone, much less a family member.
The two sisters were often seen together in their Mundelein cul-de-sac, playing and practicing cheerleader routines.
"Seemingly close. They would always be together. The older one would be taking care of the younger one, and vice versa," said Mary Ann Gryder, neighbor.
The suspect was joined in court today by her mother. She called police Tuesday morning claiming to have woken up to her sister's screams. She told 911 there was a Hispanic man standing over the 11-year-old, stabbing her. When police arrived they opened the garage door and were allowed into the house by the 14-year-old, who was crying, with blood on her arms. Her hair was wet and so was her shirt.
Police then went upstairs and found the 11-year-old stabbed to death, a knife on her bedspread next to a green shirt. The 14-year-old was then taken to the Mundelein Police Department and later confronted with the possibility of DNA testing to identify whose hair was found in the 11-year-old's hand, police said. She then admitted she stabbed her sister because the younger sibling was "ungrateful" for all she had done for her, prosecutors said. She tried to cover up her crime with a shower and the call to 911, prosecutors said.
The 14-year-old was visibly distraught in court Wednesday, but she was still calm and composed. She told the judge she understood everything that was being said to her. Police say she told them exactly what happened Tuesday morning - that she woke up just before 8 a.m., stayed in bed for about 10 or 15 minutes contemplating what she was about to do. She walked downstairs, got a knife, walked back upstairs to her half-sister's room, where the half-sister was sleeping and then stabbed the young girl. The medical examiner says the 11-year-old girl suffered about 40 stab wounds, some of them were inflicted when she was apparently trying to defend herself, so she did wake up during the attack.
"We are very confident this is an isolated incident. There are no other concerns to the neighbors or the community," said Chief Eric Gunther, Mundelein Police Department.
At a Wednesday morning press conference, Mundelein Mayor Steve Lentz said the situation is difficult for the town and asked for prayers.
"Please pray for us. This is quite a tragedy. Just pray first of all for the family," said Mayor Steve Lentz, Mundelein.
It's unknown if the girl will be charged as an adult. The two girls were the only ones home when police arrived.
Schools were briefly put on lockdown after the 911 call was made at 8:26 a.m. Tuesday; the lockdown lasted approximately 15 minutes. Police say they believe the murder happened shortly before the 911 call was made from the home in the 1600-block of Woodhaven.
Police said the parents were notified Tuesday.
The immediate cause of the death of the victim was "multiple sharp force injuries," which were "consistent with a fillet knife," said Lake County Coroner Thomas Rudd.
Rudd said there were approximately 30 stab wounds to the girl's neck, chest and arm. In addition to the 30, there were three incise wounds, which were consistent with a slice or slash, on her face. There were another four defensive wounds to the palm of her left hand and a single defensive wound to the palm of her right hand, Rudd said. The girl had "bruises and punctate wounds" on the right part of her chest, indicating that the tip of the knife was pushed on it, but did not enter the skin, he said. Additionally, there were knife wounds that "nicked" the carotid and right internal jugular vessels, Rudd said. There were two more stab wounds to the left upper lung, he said. Finally, the girl suffered superficial bruising to her extremities, he said.
The younger sibling attended St. John School in Libertyville, where grief counselors are helping students.
Lurie Children's Hospital psychiatrist Colleen Cicchetti has never met the sisters, but has studied adolescent violence.
"At that moment it was probably pure rage. We don't know for sure, but anyone, even a stranger, to hurt someone that repeatedly is unusual. I don't think we can know all what drove it," said Dr. Cicchetti.
Police did not release the name of the victim or the name of the girl charged with murder because of their ages but said the murder was "an extremely tragic event that shook even the most seasoned investigator."
On Wednesday night, the 14-year-old was being held in juvenile detention.
The AP and Sun-Times Media Wire contributed to this report.