It happened around 11 a.m. on the 2700 block of West 4th Street.
Officers arrived to find the teen critically wounded on the second floor with a gunshot wound to the head.
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Investigators say they believe it was the victim's 15-year-old sister who pulled the trigger.
"My detectives are working this case, putting it together. Our main point is where did the firearm come from? But we are going to find out," said Chester Police Commissioner Steven Gretsky.
Police tell us they are still trying to determine if anyone else was home at the time of the shooting, but the 15-year-old girl is in police custody Wednesday night.
Commissioner Gretsky says it's unclear whether the shooting was an accident.
Still, regardless of any other facts that surface in this case, the result is a family that is changed forever.
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"You have two siblings," he says, "you have a male listed as critical and you currently have a female who is currently being detained. My thoughts and prayers go out to the family."
Neighbors we spoke with agree.
"We haven't had any problems, you know, around here, in a long time. When I heard this, I just think it's so sad," said Francis Cooper.
Investigators say the weapon involved has been recovered, but they're still not sure about the ownership or if it is legally registered.
Meanwhile, the 16-year-old victim remains in critical condition at Crozer Chester Medical Center.