SINGAPORE - Two children of a wealthy widow, who each received a smaller inheritance than their other three siblings, challenged the validity of her will, alleging that their mother's bipolar disorder had affected her mental capacity to make a will.

The mother of five, who went from being one of a businessman's four mistresses to the matriarch of the family, died in November 2016, a few days before her 81st birthday.

In her May 2002 will, she gave her eldest son, second son and youngest son one share each of her estate, while her only daughter received a half-share.

Her third son, a father of three who did not hold a job for most of his adult life, got just $10,000.

In a written judgment on Tuesday (Dec 22), the High Court held that the will was valid, ruling in favour of the woman's eldest and second sons, the executors of the will who were sued by their two siblings.

Judicial Commissioner Tan Puay Boon found that the woman's bipolar disorder was in remission at the time, rejecting the claim of her third son and daughter that their mother lacked testamentary capacity.

He also rejected their claim that their mother made the will under the "undue influence" of their eldest and second brothers.

The judge said the evidence showed that she "exercised a substantial degree of free will and independence in her daily affairs" at the time of making the will.

The woman, a Cantonese-speaking former lounge hostess, left an estate described by the judge as "substantial".

It included a 856.9 sq m family property in Tanjong Katong worth more than $10 million.

If the will had been found invalid and her estate divided equally among the five siblings, the three favoured sons would each get several million dollars less.

She also provided for her husband's children from his first marriage, leaving his biological son $10,000 and two adopted children $5,000 each.

In a 123-page judgment, the judge delved into the woman's relationship with each family member, the events surrounding the making of the will, her medical history and the expert opinion of four psychiatrists.

The document did not disclose the names of the family members, who were instead assigned pseudonyms.

The woman, who was made to work at night clubs since she was a teenager, became the third mistress of a man described as a womaniser.

She had five children with him, and after his first wife died, he married her.

Although they were an affectionate couple, she had bouts of jealousy, suspecting her husband of having affairs with the domestic helper and their daughter-in-law.

She was first diagnosed with bipolar disorder in 1978.

In 1997, she was found in the family home wielding a knife and screaming for no reason.

Over the years, she was treated for her mental illness on multiple occasions.

After her husband died in January 2002, she told her lawyer to draft a letter to evict her third son, Derek, from the family home. She also told the lawyer to draft a will.

In April 2003, she went into a coma from an overdose of lithium, which was prescribed to treat her bipolar disorder. She never fully recovered and lost her mental capacity.

In the judgment, the judge said the woman's rationale for giving Derek only $10,000 was "logically explicable", given their poor relationship.

Derek, who lived in the family home with his wife and children, never had to get a job because his father gave each son a significant share of the family companies, with yearly dividends amounting to $100,000 to $200,000.

Multiple witnesses said the woman frequently complained about Derek's unemployment and his refusal to move out.

She was so bent on getting him to move out that she used a changkul, a digging tool with a long handle, to smash the windows of his bedroom in 1999.

The judge said it was also explicable that the woman gave her daughter Celine less than what the other three sons each received, but more than what Derek received.

He said there was sufficient evidence that the woman held conservative views on gender roles and subscribed to traditional values of her generation that favoured sons over daughters.

The judge noted that Celine's decision to marry a Caucasian man upset her mother, who threatened to disown her and refused to attend her wedding. Mother and daughter eventually reconciled after Celine gave birth to her first child.

"Whatever legacy that children may expect to receive under the will of a parent who has passed on, it should be remembered that a testator has full autonomy on how to dispose of his estate," said the judge.

"A parent is entitled to have favourite children and least-favoured children, in accordance to his or her subjective preferences."

Unless it can be shown that the will is invalid owing to a lack of testamentary capacity, undue influence or some other reason, the parent's wishes have to be respected and followed, said the judge.