Police say further charges are likely.
The accused lived on Lemnos Ave, police confirmed. He was known to the victims but was not related to them.
Officers found the accused man upon arriving at the avenue, then entered an address and found the dead woman in a bedroom along with the injured girl, McKee said.
The two victims were found in an upstairs room. They did not live at the address but knew one of the occupants.
Residents of the avenue include foreign diplomats.
Around 20 police are working on the investigation.
A scene examination began Saturday morning and officers could be at the scene for up to a week.
Police were called to a property on Lemnos Ave around 1.10pm Friday.
No one else was being sought, police said.
