The Court of Appeal has granted Christchurch woman Helen Milner's bid for further scientific testing of her dead husband's body parts.

Milner dubbed the "Black Widow", was convicted of the murder of her second husband Phil Nisbet by poisoning him with crushed sedatives in his food in 2009.

Milner was sentenced to life in prison with a minimum non-parole period of 17 years.

The court's decision today has left Nisbet's sister Lee-Anne Cartier devastated and in tears.

Nisbet's death was originally ruled a suicide, but Coroner Sue Johnson raised suspicions which then led to police reviewing the case and ultimately launching a homicide investigation.

Milner, 55, has so far unsuccessfully appealed against her murder conviction.

The body samples are held by the Institute of Environmental Science and Research (ESR).

Milner hopes the test results may support an application to the Governor-General for an exercise of the royal prerogative of mercy, which provides a special avenue for criminal cases to be reopened "where a person may have been wrongly convicted or sentenced".

The decision released by the Court of Appeal today said the right to apply for exercise of the prerogative of mercy was a vital backstop against miscarriage in cases involving fresh evidence.

"We do not see it as the function of this or any other court to prejudge an application for exercise of the prerogative. The courts should not, therefore, decline an application for scientific testing of evidential material in support of a prerogative application, unless it is wholly speculative and unsupported by cogent expert opinion."

The decision noted the Crown accepted this was not such a case and it did not oppose testing but rather took the view that it should occur under appropriate conditions.

Cartier said the decision was a huge blow to her and her family.

"The family is devastated that the Court of Appeal are allowing her to violate the last physical part of Phil, on a completely outrageous and unfounded goose-chase," she told the Herald.

"We are utterly disappointed that the court wasn't prepared to apply any filter of common sense but rather they have awarded a murderer the automatic right to these, Phil's last human remains over the rights of us, his family."

Cartier said the family felt tormented by the case which had now stretched on for more than a decade.

"When is this going to end?

"We could move forward in our lives instead of just waiting for the next thing to deal with. I just wanted to be able to cremate those remains and put them with Phil and move on. How are we ever meant to start to heal and move forward in our lives when we're constantly dragged back into this mess?"