Alain Delon, who starred in classics such as "Swimming Pool", "The Leopard" and "Purple Noon", suffered a stroke in 2019.
Alain-Fabien Delon, 29, accused his sister Anouchka, 33, of taking advantage of their father's declining capacities in a complaint filed on December 22 in the central Loiret region of France, local prosecutor Jean-Cedric Gaux said.
Alain-Fabien on Tuesday evening accused his sister of "pretending to be the favourite, someone so concerned and worried about her father's health.
"If you were so worried about his health, you would have been there for Christmas," the model told broadcaster BFMTV.
On Sunday he published a recording on Instagram he said was a conversation between his sister and father, in which a woman speaks to a man she calls "dad".
'Not daughter of a wallet'
"You're being taken for a fool... (and) me an idiot manipulating her father," the woman's voice says, in what Alain-Fabien says is a recording he made on Friday placing his phone under a napkin.
The feud went public after Delon's oldest son Anthony Delon, 59, told the Paris Match magazine last week that his father was in a "weakened" state.
Anthony, the son of Delon's former wife Nathalie Delon, also took aim at his half-sister Anouchka, the daughter of Delon and Dutch model Rosalie van Breemen.
He accused her of not informing him and Alain-Fabien, also the son of Van Breemen, that their father had failed five cognitive tests in Switzerland between 2019 and 2022.
Delon senior, through his lawyer, said he was "shocked" and would file a complaint for defamation.
The next day, Anthony accused his half-sister of wanting to take their father to Switzerland, a country of which the actor has been a citizen since 1999, against his will to avoid "huge tax" on inheritance.
He claimed that she would be receiving a larger share of their father's fortune.
Anouchka retorted on Sunday evening on the TF1 channel that she wanted to take her father there for medical treatment, and was not "the daughter of a wallet".
Delon is estimated to be worth millions, selling some 80 works of art at auction in June for more than eight million euros ($8.7 million).
