The case centred on the role that senior lawyer Lee Suet Fern played in preparing and executing founding prime minister Lee Kuan Yew's last will signed on Dec 17, 2013.
In January last year, the Attorney-General's Chambers (AGC) referred a case of possible professional misconduct involving Mrs Lee to Singapore's Law Society.
The AGC noted that Mrs Lee appeared to have prepared the last will of the late Mr Lee Kuan Yew and arranged for him to execute it despite the fact that her husband, Mr Lee Hsien Yang, was a beneficiary and had his share of the estate increased under the last will.
The case was later referred to a disciplinary tribunal appointed by Chief Justice Sundaresh Menon, comprising Senior Counsel Sarjit Singh Gill and lawyer Leon Yee Kee Shian. The tribunal found Mrs Lee guilty of grossly improper professional conduct in her handling of her father-in-law's will.
It said in its 206-page grounds of decision that the facts exposed an "unsavoury tale" of how Mrs Lee and her husband had misled the senior Mr Lee to sign a new will without the advice of his usual lawyer, Ms Kwa Kim Li, who had prepared all six of his previous wills.
The senior Mr Lee, who died on March 23, 2015, was then aged 90 and in poor health.
The tribunal held that Mrs Lee had acted as Mr Lee Kuan Yew's lawyer as she was the only lawyer responsible for the last will after Mr Lee Hsien Yang cut Ms Kwa out by not including her in an e-mail thread urging his father to proceed with the execution.
Mrs Lee herself had told the AGC in December 2018 that her father-in-law had given her instructions to have his last will engrossed, or finalised, the tribunal noted.
This was also Mr Lee Hsien Yang's position in a letter to a ministerial committee set up to look into options for his father's Oxley Road house.
However, Mrs Lee later changed her position in her affidavit.
She initially did not deny drafting the will but later said her husband had forwarded the draft will to her, even though neither of them could produce the e-mails to prove this.
In acting as Mr Lee Kuan Yew's lawyer, she failed to advance his interest unaffected by that of her or her husband, the tribunal found.
It described Mrs Lee as a "deceitful witness who tailored her evidence to portray herself as an innocent victim who had been maligned", adding that her husband's conduct was "equally deceitful".
The matter was then referred to a Court of Three Judges, the highest disciplinary body for professional misconduct among lawyers.
In August, the court reserved judgment after a virtual hearing.
During the hearing, the Law Society, represented by lawyer Koh Swee Yen, had argued that Mrs Lee had been involved in the will despite knowing her husband stood to gain from it.
It also said that the lawyer of 37 years had hurried her father-in-law through the process of signing it without the advice of his usual lawyer.
Countering the Law Society's arguments, Senior Counsel Kenneth Tan and Professor Walter Woon, a former attorney-general, said Mrs Lee was acting out of affection and concern as a daughter-in-law in assisting Mr Lee Kuan Yew, and was merely performing an administrative role.
They added that there could not have been any conflict of interest because Mr Lee Kuan Yew, "a brilliant lawyer", was fully aware of what he wanted and had consented to Mrs Lee handling the will for him. They called for all charges to be dropped.