The younger siblings of Singapore’s prime minister, Lee Hsien Loong, have launched a scathing personal and political attack on him, saying they have lost confidence in him as a leader and accusing their “big brother omnipresent” of abusing his power.

Lee Wei Ling and her brother Lee Hsien Yang said in a public statement released in the early hours of Wednesday that they were “disturbed by the character, conduct, motives and leadership” of the premier and his wife.

Their criticism appeared to focus on a dispute over the fate of the family home, the influence of the first lady on government, and allegations that the prime minister had unannounced political ambitions for his son.

“We feel extremely sad that we are pushed to this position,” they wrote. “We have seen a completely different face to our brother, one that deeply troubles us.”

Lee Hsien Yang, younger brother of Singapore’s prime minister, Lee Hsien Loong. Photograph: Roslan Rahman/AFP/Getty Images

Lee Hsien Loong later said he was “deeply saddened by the unfortunate allegations”.

Since the death of their father and former leader, Lee Kuan Yew, two years ago, the siblings said they had “felt threatened by Lee Hsien Loong’s misuse of his position and influence over the Singapore government and its agencies to drive his personal agenda”.

“We feel big brother omnipresent,” his siblings said, adding that Lee Hsien Yang, chairman of the Civil Aviation Authority of Singapore, and his wife now felt “compelled to leave the country”.

Lee Kuan Yew ruled from 1959 to 1990 and is credited with turning the former colony – which is poor in resources – into south-east Asia’s wealthiest nation, with a dynamic tech industry. His eldest son and current leader also retains a firm grip on power and widespread popularity, winning 83 of 89 seats in a 2015 election.

Wednesday’s statement did not explicitly state any dangers posed to siblings but said that the PM was driven by a desire for “power and personal popularity”, milking his father’s legacy for political purposes.

“We feel hugely uncomfortable and closely monitored in our own country. We do not trust Hsien Loong as a brother or as a leader. We have lost confidence in him,” the statement read. “We also believe, based on our interactions, that [the PM and his wife] harbour political ambitions for their son, Li Hongyi.”

The comments by Lee Wei Ling, a neurosurgeon, and Lee Hsien Yang also took aim at the first lady, chief executive of the state investor Temasek.

Lee Wei Ling with her father and mother Kwa Geok Choo in 2003. Photograph: David Loh/Reuters

They said that unlike their mother, who acted outside the limelight as a private adviser, Ho Ching had amassed huge influence that extended “well beyond her job purview”.

The extraordinary display of disapproval rushed through social media in Singapore, where open criticism of political leaders is discouraged and can lead to prison sentences.

The bulk of the six-page statement focused on a dispute over the family home, 38 Oxley Road, referencing how their father had asked for it to be demolished upon his death, which they said was part of his long-held beliefs against monuments and self-aggrandising.

“However, we believe that Hsien Loong and Ho Ching are motivated by a desire to inherit Lee Kuan Yew’s standing and reputation for themselves and their children,” the statement read.

“The simple truth is that Hsien Loong’s current popularity is tied to Lee Kuan Yew’s legacy. Preserving Lee Kuan Yew’s house would allow Hsien Loong and his family to inherit a tangible monument to Lee Kuan Yew’s authority.”

They said Lee Hsien Loon had opposed Lee Kuan Yew’s wishes and “expressed plans to move with their family into the house as soon as possible after Lee Kuan Yew’s passing”.

The prime minister’s statement said he was “very disappointed that my siblings have chosen to issue a statement publicising private family matters”.

Denying the allegations, including harbouring any political ambitions for his son, he said differences among siblings should “stay in the family” and the statement had “hurt our father’s legacy”.

Although rare, the ruling family have had public spats in the past – Lee Wei Ling said in April 2015 that the government had tried to use the first anniversary of Lee Kuan Yew’s death as “hero worship” and that her brother was “abusing his power”.