Everything depends what's important: fulfilling brother's wished or money.
I can't give specific legal advice here, but speak of generally along the lines of these issues.
When it come to disturbing gravesites in contradiction of existing covenants, cemetery officials are probably most concerned with MONEY.
Is cemetery worried that brother's family will try and recover the money brother expended on the burial plot?
If the plot is already purchased, the deal is done and cannot be undone unless brother lacked contractual capacity at the time he bought the plot. If the family's issue is not about money, but only about fulfilling brothers burial wishes, assurances of that, even to the extent of a legal release precluding any future attempt to vitiate the burial plot deal.
Obviously, if the family intends, or may at some future time, to try to recover the money for the burial plot, they should not sign a release of their right to do so.
The other money pit for the cemetery would be cemetery's potential liability for intentional or negligent infliction of emotional distress for disturbing corporeal remains -- i.e. mishandling corpses. Intentional mishandling is subject to liability as an intentional tort, and even negligent mishandling generally carries inevitable and automatic liability in negligence, and so the cemetery is probably seeking to insulate itself from all such tort liability (lawsuits!).
Are there other siblings or relatives that might object (or file a lawsuit) to the parents' gravesite being disturbed to the point of interring brother's ashes?
If not, cemetery officials may accept affidavits -- along with what illustrations of brother's intent available -- to that effect to convince the cemetery they will not be subject to any tort liability from any living family members.
If the prime concern is fulfilling brother's wishes, then signing releases of liability may satisfy the cemetery.
HOWEVER, if the family wishes to preserve the issue contesting the purchase of the burial plot or does not want to relinquish potential causes of tort action, then obviously legal releases giving up llegal rights should NOT be signed.