Indigenous woman Veronica Philomena Lockyer, 33, and seven-month-old Adell Sherylee Partridge were last seen in late 1998, the Corners Court was told on Wednesday.
Deputy State Coroner Sarah Linton told the Lockyer family after hearing evidence it was likely she would find that their mother and sister "died a long time ago".
For about 20 years, their family assumed the transient mother-of-six had made a new life elsewhere until their disappearance was reported to police in 2018.
But an intensive Australia-wide police investigation found no trace of the pair - a bank account had not been touched and there were no government records to indicate "proof of life".
Ms Lockyer's partner before she disappeared, Craig Partridge, said he left the pair in a Perth shopping centre car park after an argument.
He was initially a person of interest during the police investigation after it was learned the relationship was violent but no charges have ever been laid.
Mr Partridge denied having anything to do with the duo's disappearance in the months before he was admitted to hospital for psychiatric treatment after reportedly hearing voices telling him to harm himself and others.
"I don't know where she is Your Honour," he said from the witness box.
"I've been looking and looking and I can't find her."
Mr Partridge raised the couple's other daughter, Donna Lockyer, who was about two years old when her mother disappeared, until she was 14.
Now a mother of three, she told the court her father was a violent man during her childhood in the small wheatbelt town of Merredin.
She said Mr Partridge told her Ms Lockyer had abandoned her and gone to live in Port Hedland, and that she had never seen a photo of her mother until police showed her one during the investigation.
"He wouldn't tell me about my mother," she said sobbing.
"I would always ask about family and he would say 'you don't want to go there'.
"Craig stripped me from everything. Stripped me from my language, my culture."
Donna Lockyer said she raised the alarm with police after she learned she had siblings amid a push as a young adult to find out why her mother "didn't want" her.
Her brother, Adrian Clinch, who was about nine years old the last time he saw his mother, alleged Mr Partridge had stopped Ms Lockyer seeing her family in the Pilbara before she disappeared.
He also said his mother was a heavy drinker and he remembered seeing her with two black eyes during a visit to the community where he lived as a child.
Outside court, Mr Clinch told reporters he suspected his mother had met with foul play and that the inquest hadn't provided the answers the family had hoped for.
Donna Lockyer said the family was now considering hiring a private detective to investigate the matter and would continue to raise public awareness.
"I'm currently on TikTok and I post a lot of things about my mother and sister to also circulate (their story) through the web," she said.
"We are not going to give up until we bring the home, alive or deceased.
"Until we have our answers we will not let this dwindle away."
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