She tried to stay friends with Milner and sympathise with her, in the hope she would "let something slip'', it emerged in court today.
Mr Nisbet, 47, was found dead in the early hours of May 4, 2009 in a case that police originally ruled suicide.
Milner denies murdering her second husband by giving him a fatal overdose of the anti-histamine and sedative Phenergan, and possibly finishing him off with a pillow over his face.
The Crown said she was unhappy in her marriage and motivated to murder by the prospect of cashing in the $250,000 life insurance policy.
Milner plotted the best ways to kill her husband; buying drugs under false names, asking friends for views on poisoning methods, and even offering to pay $20,000 for a hit man to kill Mr Nisbet, it is alleged.
Defence counsel Margaret Sewell put it to Ms Cartier under cross-examination today that she became a "sort of detective'' after seeing the alleged suicide note on June 6, 2009.
Ms Cartier accepted that she had.
"My brother had died and I needed to know the truth. And I think everyone here in this room would've done that too.''
Within four months of the death, she had talked to Milner's workmates at Christchurch-based grounds maintenance firm GSL.
They told her Milner had been asking about rat poison and whether it would kill someone.
She passed the information to police on August 26, 2009.
Ms Cartier - who lived in Queensland, but returned several times in 2009 and 2010 - also spoke to Milner's neighbours, who told her they'd seen a bedroom light on the morning Mr Nisbet died.
While she was over for a visit a month after the death Milner gave Ms Cartier a sim card and phone to use.
She wrote down all the contacts on the sim card and handed them to police.
"Then you preceded to contact a lot of people - anyone you thought might be associated with this case?'' Ms Sewell asked, and Ms Cartier agreed she did.
The trial, before Justice David Gendall, continues.
HUSBAND POISONING TRIAL - A SNAPSHOT OF EVIDENCE DAY BY DAY
* DAY ONE
Opening address from the Crown, alleging that Helen Milner was motivated to fatally poison her husband by the prospect of a $250,000 life insurance pay-out. She plotted the best way to do it, the Crown alleged, asking friends for views on poisoning methods, and even offered to pay $20,000 for a hit man to get Mr Nisbet out of the picture. And a new man had moved in to the family home within weeks of his death.
* DAY TWO
Milner told workmates of fears that her second husband was trying to kill her. She also asked them if Mitre 10 sold rat poison and whether it would kill a person. First police officer on the scene of Mr Nisbet's sudden death raised concerns over the "convenience" of his wife receiving a texted suicide note in front of him and queried her hysterical grief, saying it "appeared to be acting".
* DAY THREE
Workmates of Milner say she was unhappy in her marriage and talked of "getting rid of him". They laugh it off, nicknamed her the 'Black Widow' and joked about not eating her muffins. A 111 call where a hysterical Milner reported her husband's death was played to the jury: "My husband's dead...", she said in the call made at 5.46am on May 4, 2009.
* DAY FOUR
Court heard Milner was quizzed by Mr Nisbet's family at a coronial inquest over the validity of a suicide note she claimed to have found. A cop tells of finding evidence Milner bought an engagement ring with her new man just two months after her husband died. Makes claims to his sister Lee-Ann Cartier that Mr Nisbet had been a male prostitute, had affairs, and fathered illegitimate children.
* DAY FIVE
Resumption of trial delayed after the diabetic Milner had a "health issue", Justice David Gendall tells the jury.
Under cross-examination, Ms Cartier admits she suspected her brother's death wasn't a suicide a month later after seeing a "suicide note" Milner claims to have found. She spends the next two years doing her own "detective work", feeding information to police. "It didn't add up," she said.
