Police did not treat the disappearance of Arlene Arkinson with the seriousness it deserved, her sister said.
It took detectives six weeks to search chief suspect Robert Howard’s home, giving him ample opportunity to destroy evidence, Kathleen Arkinson added.
She complained to the Police Ombudsman watchdog about the conduct of police.
“They were not prepared to give it the credence or the seriousness that it deserved,” she said.
Her own home was searched twice, her garden dug up and the witness claimed she had been placed under house arrest for several days.
She cast doubt on the testimony of three people last in Arlene’s company.
The witness said Howard was a convicted sex offender and she found out later that he had been on a night time 7-7 curfew.
“At the time police had a chance to arrest him for that but they did not.”
She said it took six weeks to detain him.
“He would have had ample opportunity to destroy evidence from his car, which I don’t believe was searched.”
Police swooped on the home of Kathleen Arkinson twice, once in April 1996 and once ahead of the 2003 trial of Howard for murdering London schoolgirl Hannah Williams.
The first time they dug up her garden.
“They came in with sledgehammers and opened up the whole house. They had me in for two or three days because I could not get out, I needed nappies for the wee one.
“They said ‘you are going nowhere, you are under arrest’.”
She said when she protested they called a doctor who gave her a “jag” of something she did not understand.
Unfounded rumours of her involvement in the murder persisted for years.
“It was all everybody was talking about, the whole world was talking about it.”
Kevin Rooney, a barrister for the police, said detectives had received information a short time before the 1996 raid, on April 14, from an anonymous source.
A public appeal for information yielded more information. That person then made contact with the police.
“On the basis of that information your house was searched but nothing was found.
“Any opportunity that they might be given, however remote it might be, to find out anything that may be of assistance to establish where Arlene was and how she died, would you accept that it would be important to police?”
The informant’s identity is protected but when Ms Arkinson protested coroner Brian Sherrard said he had not finally decided yet whether it could be revealed.
Arlene was not particularly resourceful and did not have much money, her sister said.
She had drank since she was 14. She was “interfered with” at a young age, lawyer Frank O’Donohue said.
But she was a brilliant painter and a happy girl who loved make up and dancing.
Her mother died when she was young and her father developed alcoholism. Arlene went to live with a sister, Anita.
Kathleen Arkinson said: “She was looking for attention because mammy was not there.”
She recalled the last time she saw her alive, as she left the home for Bundoran in the company of friends.
“She had one punt (Irish pound) in her pocket. She had a top on but no cardigan or a jumper.”
She said children later told her they saw her sister getting into Robert Howard’s car.
Sobbing in the witness box she added: “She was brilliant at painting, I still have her painting at home with her name on it.
“I believe Arlene was murdered by Robert Howard.”