Francisco Marcolino married his maid of decades, Rita Monteiro, 52, in northern Portugal on 4 May.
But three children from his previous marriage claim she is eyeing his €2m (£1.7m; $2.2m) estate and that he is not of sound mind.
"If he had wanted to marry, he would have done it when he was capable," his son told Portuguese media.
The children are trying to annul the marriage on grounds of incapacity - though a fourth is refusing to join the action and has not commented.
Ms Monteiro was hired by Mr Marcolino's wife 30 years ago, two years before she died.
According to Manuel Marcolino de Jesus, one of three children who have filed the lawsuit, his father's health has sharply deteriorated over the past few years.
His father has made several emergency trips to the local hospital, where his files say he suffers dementia and is totally dependent, Manuel Marcolino said.
"They have lived in [Ms Monteiro's] house since he became incapable five years ago, and that is when she started to take over everything," he told Portugal's CM Jornal.
"Since then, more than €319,000 has gone missing from one [bank] account and more than €200,000 from another," he alleged.
Last December, the three lodged two criminal cases of abuse of trust against Ms Monteiro and listed Mr Marcolino's assets. A psychological assessment was under way to see whether Mr Marcolino should be appointed a representative to manage his affairs.
But they say they were caught by surprise when they found out their father had married.
They say officials at a register office 80km (50 miles) away refused to marry the pair, forcing them to travel to another one 150km away.
Portuguese news agency Lusa has repeatedly contacted Ms Monteiro's lawyer, but her lawyer says she has "no questions to answer".
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The children of a 101-year-old man have legally challenged his marriage to his ex-maid, who is just over half his age.