Skye Mason, who is charged with taking baby Alyssa Barker from her North Auckland home, had a childhood of drugs, broken homes and teenage pregnancy.
Her sister Calah Mason, 26, told the Herald on Sunday: "Skye has never really had a family, a father figure or a mother figure ... She's just a lost soul."
Calah said Skye, 27, had three sons aged between 3 and 11, but had custody of none. She understood two lived with their fathers and one was in foster care.
Calah said Skye would never have harmed a child.
Skye was the oldest of four girls born to Amanda Mason. Skye and Calah had different fathers and the younger sisters were born to a man who gained custody of all four when Skye and Calah were little.