The deadly shooting took place in the village of Hennezel in the Vosges area of eastern France on Sunday, according to French news site 20 Minutes.
According to police the boy was playing with some arms at the table and did not realise that one of them was loaded.
“One member of the family got out several weapons, without knowing, or without warning the others, that some of them were loaded,” the police commander in charge of the inquest told AFP.
“At around 10.30pm the 14-year-old boy picked up an automatic pistol and accidentally let off a shot that hit his twin brother in the thorax.
“Firefighters who were called attempted to save his life, but it was in vain,” said the police chief.
Searches of the premises were still being carried out on Monday as part of the ongoing investigation.
No arrests have been made.
France has fairly strict gun laws but firearms can be bought with special hunting or sports licenses.
The licenses need to be repeatedly renewed and to gain one involves psychological testing.
In 2012 the French government estimated that there were at least 7.5 million guns legally in circulation.