Leniata had been accused of sorcery by relatives of a 6-year-old boy who had recently died in a hospital. Ware and Watea are believed to be the boy's mother and uncle, police said in a statement.
The two were among more than 40 people who were detained last week in connection with Leniata's slaying. The others were eventually released due to lack of evidence, but police said more arrests are expected.
In rural Papua New Guinea, witchcraft is often blamed for unexplained misfortunes, but the brutal killing was met with outrage across the South Pacific island nation, drawing condemnation from the prime minister, police and diplomats.
Police said the hundreds of onlookers, many of whom were children and teenagers, were powerless to stop the mob who participated in Leniata's killing in the Western Highlands provincial capital of Mount Hagen. Police officers were also among the spectators, but were outnumbered and couldn't save the woman, national police spokesman Dominic Kakas said. An internal investigation is under way into the police's actions at the scene.
Murder in punishable by death in Papua New Guinea, although no one has been hanged since the country became independent in 1975.