A woman who says she was written out of her aunt's will after she was unable to assist the elderly woman to end her life will not receive any of the $1 million estate.

Beryl Lee Hordern died in an Elizabeth Bay nursing home at the age of 86 in September 2014, leaving her entire estate to her friend and carer Cynthia Carr.

But her niece Ann Richardson successfully contested that will in the Supreme Court last year, arguing that her aunt "lacked testamentary capacity" when she signed the 2004 will.

Ms Carr appealed that decision in the Court of Appeal, which on Tuesday overturned the Supreme Court judgement, unanimously ruling that Ms Richardson was no longer the beneficiary of the will and that Ms Carr was now entitled to the estate.

The Court of Appeal heard that in her 2001 will, Ms Hordern left her entire estate to Ms Richardson, but changed her will in 2004, after a falling out with her niece.

Ms Hordern had been unsure exactly how much her estate was worth, but estimated it to be more than $1 million.

Mental capacity in question

The court heard there were questions over Ms Hordern's mental state around the time she made the 2004 will and that she would give taxi drivers and her building's concierge $50 tips "without thinking".

On several occasions in December 2003, she withdrew large amounts of money totalling $150,000 from her bank account before a friend collected it all and deposited it back into her account, the court heard.

In February 2004, Ms Hordern was assessed by a social worker and found to have "moderate dementia" after a friend had raised concerns about her ability to manage her own affairs.

The court heard Ms Richardson had not seen her aunt and had not checked on her welfare since 2003, when the pair had a falling out.

It heard Ms Hordern had told her solicitor she did not want to leave anything to her niece as she had "disgraced herself with comments about my sister [Ms Richardson's mother]".

When the solicitor asked if she was sure of her decision, Ms Hordern replied: "Of course I'm sure."

"Cynthia has been a very good friend to me over a great number of years — in fact she is my only real friend."

The court also heard that Ms Hordern thought her niece, who lives in Portugal, did not need the money.

Ms Richardson confirmed that she had fallen out with her aunt, but said it was not over her feelings for her own mother.

She told the court that she had a "normal loving relationship" with her mother, who had died in 1972, and that her relationship with her aunt broke down after Ms Hordern asked her to make inquiries about how she could die by euthanasia.

Ms Richardson said she told her aunt that the Euthanasia Society said it was "not legal to do it. The society does not come out to help you".

She said her aunt had replied: "You knew I wanted it so why did you stop it? What you did was wrong. I can't believe it."

The Court of Appeal rejected the trial judge's finding that Ms Hordern was "delusional" in her reasons for changing the will and had erred in finding that Ms Hordern's decision had resulted from "an unsoundness of the mind".

It found that she was well aware of her actions and their repercussions when she made the 2004 will.