The 15-year-old from Castlederg, Co Tyrone, disappeared in August 1994 after a night out in Bundoran.
In July 2021, an inquest found she was murdered by convicted child killer and rapist Robert Howard, who died in 2015.
Howard had previously been charged with her murder in 2002 but acquitted in 2005.
Speaking ahead of a 30th anniversary vigil, Kathleen Arkinson said her sister deserved “to be found and get a decent burial.”
Heartbroken Kathleen said she believed deep in her heart that “some day Arlene will be found”.
Numerous searches, including some in vast swathes of bogland and forestry near the Donegal border, have been carried out in search of Arlene’s remains.
One of the last searches was conducted by Gardai in May 2024 in the Castlefin area.
Kathleen said she has mixed emotions ahead of the memorial in her sister’s memory.“One day I’m alright, the next I’m not – I just can’t believe it’s been 30 years,” she said.
“I’m thinking about Arlene and that night – I know she suffered, but I just wish I could’ve taken the pain.”
“[Robert] Howard killed Arlene, but he also killed a part of us as well,” she added.
An inquest in 2021 found Robert Howard, who died in 2015, murdered Arlene Arkinson
Howard, who was the last person seen with Arlene Arkinson before her disappearance, was previously charged with her murder, but was acquitted in 2005.