Bruni Tedeschi, the elder sister of Carla Bruni, said her work on 'A Castle in Italy' (Un Chateau en Italie) helped heal old wounds.
"For me work is a form of therapy," she told reporters. "I sleep better when I have the impression I've worked well during the day."
She recast actual events, including her own brother Virginio's AIDS death in 2006 and her relationship with co-star Louis Garrel, with often absurdist takes on them on screen.
"When I'm working on a different planet like this, it's like being in a carnival," she said.
"Everything is back to front, upside down. In that sense it is like therapy, it helps me to enjoy life more."
Bruni Tedeschi plays Louise, the 40-something retired actress and heiress to a Franco-Italian industrialist family that can no longer afford the upkeep on its elegant manor home in the Italian countryside and sets up in Paris.
The director's own wealthy family left Italy for France in the 1970s, fearing attacks by far-left radicals.
Once in Paris, Louise begins dating Nathan (Garrel), a man almost 20 years her junior who she hopes can give her a child.
Meanwhile her ailing brother and elderly mother cling to the family castle and their collection of Old Masters' paintings even as an accountant tell them they will have to make some major sacrifices to pay off an unexpected tax bill.
Bruni Tedeschi said she chose her mother, Marisa Borini, an actress and concert soloist who also plays piano in the film, for the part but was careful as they relived their grief for Virginio.
"I chose my mother to be in the film because she's a wonderful actress. Perhaps I make her work even harder than the other actors. Sometimes I'm more strict with her, I like being very strict with my mother," she said.
"I always thought a lot of work would help her to act in this film without feeling too much pain and suffering. The idea was to protect her, I think work protects us from what might be a psychodrama."
Borini said in notes for the film that "making a movie changes absolutely nothing" in terms of the pain of losing a son.
But she said at the same time that it was important for the family itself to tell its own story, rather than "someone else going to our family cemetery".
Bruni Tedeschi, the only female director in competition at Cannes this year, dedicated the film to Virginio.
But despite feverish speculation in France before the premiere, Carla Bruni is not depicted in the film.
"A Castle in Italy" is one of 20 films in the running for the Palme d'Or top prize to be awarded by jury president Steven Spielberg.