According to the first elements of the investigation, the woman, aged 38, was in a room and the girls, aged 9 and 12, were discovered in their room. The 40-year-old man was found hanged on the ground floor, the same source said.
A dog, apparently belonging to the family, was also found dead.
Early Sunday afternoon, the street leading to the house where the bodies were discovered, located in a hamlet near Carantec, was cordoned off by the police, noted an AFP photographer.
A criminal identification van and a fire engine were parked in front of the small independent house. It is not known if it belongs to the family or if the latter stayed there during the holidays.
No details have been provided on how the woman and the girls died, nor on the date on which the tragedy would have taken place.
The prosecution said it would issue a statement at the end of the day. The gendarmerie did not give details on the circumstances of the discovery.
Carantec is a seaside town of about 3,200 inhabitants year-round, in the bay of Morlaix, located about ten kilometers from this sub-prefecture.