Julie Hambleton, whose 18-year-old sister Maxine was killed in the 1974 attacks, said the hearing by a coroner of an application to resume the inquest was “momentous” for the families.
Twenty-one people were killed and 182 injured when the suspected IRA bombs exploded in two city centre pubs on November 21 1974.
Six men wrongly convicted of the murders – the Birmingham Six – were released in 1991 after their convictions were overturned by the Court of Appeal.
An inquest, which opened days after the bombings, was closed without hearing evidence in 1975 in response to the guilty verdicts.
Ms Hambleton told Good Morning Britain: “This public hearing today is momentous for us and if the coroner, we hope and pray, decides in our favour it will be seismic for us.
“This campaign is a campaign done in sadness and not in anger for those who are not here to fight for justice themselves. Twenty-one people were murdered in cold blood – they have no voice, they have no opinion, but we are here and we can fight for them.”
She added her family’s lives “fell apart” the moment they discovered her sister had died.
Asked why she thought police would object to the opening of an inquest, Ms Hambleton said: “We don’t know. We are going to have to wait until the public hearing opens later today to hear what their legal argument is and then our legal team KRW Law, who are representing us, will put forward our legal arguments to the coroner (Louise Hunt) in the hope she will decide to reopen the inquests that were adjourned 41 years ago.”