At first it seemed unfathomable: 11-and 15-year-old Florida sisters arrested for allegedly shooting their brother dead while he slept.

According to police documents, the siblings and their 3-year-old sister were left alone with their 16-year-old brother while their mother accompanied their father on an out-of-town trucking job on Jan. 5. The girls told police that their brother locked the older sister in a room with just a blanket and a bucket to use as a toilet. While the brother was sleeping, police say, the younger sister unlocked the door and helped the older sister get a gun, which she then used to shoot and kill her brother as he slept on the living room floor.

Then, as more information about the homicide began to trickle out, it became clear that the girls accused were not strangers to trauma and violence. In 2010, the children's aunt went to police when she found a memory stick belonging to her husband with video of him sexually molesting the oldest sister. The uncle was eventually convicted and is serving life in prison.

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And the abuse did not stop there. The family began locking the girl in a room - once for 20 days - and reportedly pulled her from school due to "behavioral issues." And among the reports released by the Columbia County Sheriff's Office is a heavily redacted document that suggests that in 2011, when the older sister would have been 11 or 12, her mother found the girl and her brother having sex in the house. But, according to the incident report, "the case was closed as unfounded and no criminal acts were disclosed."

"We treat sexual abuse of children in the family as a social and psychological problem and not as a crime - and it is a crime," says Grier Weeks, the executive director of the National Association to Protect Children.

The Florida Department of Children and Family told 48 Hours' Crimesider that they are investigating the incident but cannot reveal whether they were involved with the family prior to the shooting.

According to Daphne Young of Child Help, a non-profit organization devoted to the prevention and treatment of child abuse, 68 percent of child sex abuse victims are abused by a family member. Jennifer Marsh of the Rape, Abuse and Incest National Network (RAINN) says that 40 percent of the people who call the National Sex Abuse hotline say incest is their primary or secondary reason for seeking help.

And yet, perpetrators who sexually abuse family members can be subject to lower penalties than they would be had they assaulted a neighbor or stranger. In Washington State, for example, the Special Sex Offender Sentencing Alternative Law allows an offender to receive a lighter sentence if he or she had "an established relationship with, or connection to, the victim."

Weeks calls this "the incest loophole," and finds it mind-boggling: "The toll on a child [abused by a family member] is devastating. She was not protected by the very people who should have loved and protected her."

Even in states where no "incest loophole" exists - like California and New York, where Weeks says legislation closing the gap was approved over the past decade - RAINN's Marsh says that "incest can be particularly difficult to investigate."

Victims, she says, fear having their family torn apart: "The idea of a loved one going to prison is more horrible than dealing with the evil they already know." Because of this, victims "may be uncooperative and downplay abuse. If it has been going on a long time, they may not even see it as a crime," Marsh says.

Actress and incest survivor Alison Angrim, who played Nellie Oleson on "Little House on the Prairie," told Crimesider that she was sexually abused by her older brother for six years starting when she was six years old. When she read about the Florida case, she said the first thing she thought was: "Why were those children still in that house?"

"The police wrote off the sibling incest because they were close in age," she says. "But the question is balance of power. Apparently, her brother has the authority to lock her in a room. He wasn't a brother who was on the same footing, he was a prison warden with access to a gun. Had he kidnapped her off the street and she'd shot him while escaping, we'd call it self-defense."

The two Fla. girls were initially charged with first-degree murder as adults, but earlier this week the state's attorney announced they would both be charged as juveniles, with second-degree murder. The girls' parents are still behind bars on neglect charges. Their 3-year-old sister is in state custody.