The two babies were born on either side of the Atlantic

The siblings are also said to have a history of feuding, as heirs to a $2m inheritance.

Prosecutor Christian Girard told Reuters news agency he had ordered an inquiry into the domestic circumstances of the case.

Ms Salomone has defended her decision telling the French newspaper Le Parisien that as she was too old to continue her genetic line, she wished to pass on her brother's genes.

Ms Salomone's brother Robert provided the sperm used to fertilise two eggs donated by a Californian woman: one was brought to term by Ms Salomone, the other by the donor.

Both children are now being looked after by the French brother and sister who live in the town of Draguignan in southern France.

Click here to see the family line

Incest

The new probe follows an ongoing investigation into the case - French law forbids post-menopausal women from receiving artificial fertilisation.

Prosecutor Christian Girard said the social circumstances surrounding the case would be investigated.

He said it was "not biological incest, but social", adding that social services might intervene and place the children in care.

"It is a misuse of science. Morally and legally, it is unacceptable," French Health Minister Bernard Kouchner told French Radio on Thursday.

No qualms

However, Dr Vicken Sahakian said had no moral qualms about helping the retired teacher with fertility treatment.

Dr Sahakian, who works at the Pacific Fertility Center in Los Angeles, argued in an interview with the BBC World Service's World Today programme that the outcry over Ms Salomone's age reflected male chauvinistic society.

Dr Sahakian: Did not realise the couple were related

"On a daily basis we see couples where the man is in his 50s or 60s and a woman is in her 30s, and we certainly don't make a big deal of it, but when it's the opposite it's headline news," he said.

But he said he had been tricked into believing that the male donor was Ms Salomone's husband.

He admitted that he makes limited checks on the personal lives of his patients.

Torrid rows

Ms Salomone asked in an interview with the Le Parisien newspaper:"I may be 62 and my brother 52, but we are better able to bring up the children than a couple of drug addicts with a kid who are living off welfare. Why judge us and not them?"

The paper described the case as an "unprecedented ethical problem", and the newspaper Le Monde labelled the affair "mind-boggling and nauseating".

The members of this family ferociously hate each other and use the police to settle their fights

Police report

The infants have reportedly been born into a fraught family atmosphere.

They are heirs to an inheritance worth $2m but the brother and sister are reported to have a history of bitterness towards each other.

A 1993 police report said that members of this family "ferociously hate each other" and use the police to settle their fights.

"The situation is very worrying and is caused by family misunderstandings about future inheritance," it read.

The woman's baby was conceived in the US by artificial insemination

At one point Robert left the house and lived in a caravan where he shot himself through the chin in a fit of depression.

Their 80-year-old mother, who they live with in Frejus in the south of France, has dismissed the suggestion that the children will would become pawns in an inheritance battle.

"I think Jeanine and Robert have been reconciled and their children will be loved," she said.