The brother of Saad Al-Hilli is being questioned by detectives investigating the killing in the Alps of Saad, his wife Ikbal, mother-in-law Suhalia al-Allaf and local man Sylvain Mollier.
Mr Al-Hilli's brother Zaid Al-Hilli, who is in his 50s and lives in Surrey, has previously denied any feud with his sibling over an inheritance. He was arrested by police at an address in Chessington.
Around 100 police officers in Britain and France are investigating the killing of the family on September 5 last year as they drove through a remote area close to Lake Annecy.
French investigators came to the UK and searched the al-Hilli family home in Claygate, Surrey, in the wake of the deaths.
They said in September that they were investigating three lines of inquiry, focusing on Mr al-Hilli's work, his family and links to his native Iraq.
Annecy's chief prosecutor Eric Maillaud said last year that there would be no ''quick solution'' in finding the killers.
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It will be increasingly humid and thundery with highs of 30C or more today.
Holidaymakers in Tunisia are determined not to be beaten. But, as Neil Connery writes, it won't be easy to escape the shock of the massacre.
Drink lots of water, stay out of the sun during the middle of the day, and take particular care of vulnerable people.