"The fifth (suspect) was arrested this morning," said police spokesman Lungelo Dlamini.
Four others detained earlier would appear in court today, he added.
"Charges against them will be kidnapping, rape, and murder," he said.
The men were aged between 29 and 47. All lived in Diepsloot township northwest of Johannesburg.
A resident discovered the bodies of the girls, aged two and three, early Tuesday in a communal toilet. They had been missing since Saturday.
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Located along one of the country's wealthiest gated estates, Diepsloot testifies to the country's huge wealth inequality.
Large parts of the squalid slum have no running water and residents share pit latrines and mobile toilets.
South Africa has one of the highest murder rates in the world. Around 16,000 people are killed a year, according to official statistics.
Over half the 54,000 crimes against children reported in the period from 2010 to 2011 were sexual offenses, according to the UN children's organisation UNICEF.
In a separate case, a mother was due in court today after her two young children were found dead in a field east of Johannesburg.