Sisters Asra Abdullah Alsehli, 24, and Amaal Abdullah Alsehli, 23, were found dead on 7 June in separate beds at home in the suburb of Canterbury.
Police, who were called to the property for a welfare check, said the women are believed to have died in early May.
But despite "extensive inquiries", they still do not know how or why.
The sisters moved to Australia from Saudi Arabia in 2017 and may have sought asylum, The Sydney Morning Herald reported. Police refused to confirm this, saying they do not comment on residential status.