Officers responded around 5:30 p.m. to a house on the 2800 block of South 10th Street, where they discovered the women, each with a single gunshot wound to the head. Police said the victims were pronounced dead minutes after first responders arrived.
Police have identified the victims as 18-year-old Angelina Picano and 55-year-old Denise Grottini.
Officials say preliminary investigation indicates they were both shot by 67-year-old Janice Picano before she turned the gun on herself. A firearm was recovered at the scene.
Police said all three women were related and lived in the home. Angelina Picano was the suspect's granddaughter, while Denise Grottini was Janice Picano's sister.
Angelina Picano was a student at Horace Howard Furness High School, according to the School District of Philadelphia.
"Death is rarely something that is easy to accept, especially when it involves a young person, particularly during these times as our city grapples with heightened gun violence," the school said in a statement.
The school plans to have counselors on site Wednesday and Thursday.
Neighbors stood outside in the rain as loved ones gathered near the home. Resident JoAnna Schofield said the violence was difficult to comprehend.
"I'm very sorry...I can't even imagine it. I can't even imagine it," Schofield said.
The shooting left the typically quiet neighborhood shaken.
"I'm shocked. You just don't hear about things like that in this neighborhood. It's just a very quiet, nice neighborhood," Schofield said.
Police have not revealed a motive.
The shooting remains under investigation by the Philadelphia Police Department's Homicide Unit.