The daughter-in-law of Singapore's founding father has been found guilty by a disciplinary tribunal of professional misconduct over her involvement in preparing his will, which is at heart of a feud between the city-state's first family.

The latest development in a long-running saga could sow further discord among the prime minister and his siblings — whose father Lee Kuan Yew co-founded the party which has ruled the island nation unbroken since independence — just as an election looms.

The family feud centers around the fate of the old house of Singapore's first premier and the father of Prime Minister Lee Hsien Loong.