When 87-year-old Chung Khin Chun decided to leave all her assets to him, she "knew exactly what she was doing", insisted Mr Yang Yin.
The Chinese former tour guide said that in 2010, when her lawyer drew up a new will for the wealthy widow, she was first made to undergo a medical examination.
The doctor found that Madam Chung, whose assets were worth around $40 million, was "fully aware of what she was stating in the will".
The widow, who this year was diagnosed with dementia, was also examined by a doctor accredited by the Office of the Public Guardian in 2012 before appointing him her guardian, said Mr Yang.
The doctor certified that Madam Chung had the "requisite mental capacity" at the time, and fully understood the implications, Mr Yang said.
The Lasting Power of Attorney (LPA) granted to him that year gave him full control over her assets, which include a $30 million bungalow in Gerald Crescent.
"Madam Chung knew her actions would attract unhappiness from her relatives and she executed the LPA and the will to protect me and my family and to give us the assurance we would be provided for," said Mr Yang, who denied splurging on expensive hotels while on overseas trips.
As for allegations that Madam Chung "lives frugally nowadays", he said the widow is "getting on in years and does not lead such a lavish lifestyle any more".
"This does not mean, in any way, that Madam Chung is being treated badly. It is just a change in circumstances," he added.