Anne Morgan was the last member of his family to see the County Down teacher alive before he was murdered and secretly buried by republicans in 1985.
Investigators from the Independent Commission for the Location of Victims' Remains (ICLVR) have been searching an area near Rouen since Monday.
Ms Morgan was shown the site by the chief investigator on Friday.
She appealed for anyone with information about her brother to come forward.
"If you know anything about Seamus' disappearance, if you know anything about this site in France, please come forward, even at this late stage," she said.
"It would be beneficial for me, for my family, for all the ones who are at home in Newry, please come forward.
"If anyone else has information concerning any of the other families we need that information and the commission needs that information to follow through with other searches."
This latest search began in a forest at Pont-de-l'Arche after new information on the whereabouts of Mr Ruddy was passed to the ICLVR from the Irish Republican Socialist Party (IRSP).
The group has close links to the Irish National Liberation Army (INLA), which killed Mr Ruddy.
Sources in the IRSP claim Mr Ruddy went willingly with INLA members from Paris to the wooded area, where there was an arms dump.
What happened among the group is not clear but Mr Ruddy was shot dead. He was buried in a shallow grave.
Sources claim that INLA members returned to the spot the following day, removed the arms cache and buried Mr Ruddy where it had been been.
It is understood that former INLA and IRSP members travelled to northern France last year to try to pinpoint the spot where the Newry man was buried.
Ms Morgan has said she knows the names of the people who killed her brother.
There have been previous searches for his remains, but the latest information is thought to give Mr Ruddy's family their best hope yet of finding him.
The Disappeared are those who were abducted, murdered and secretly buried by republicans during Northern Ireland's Troubles.
Mr Ruddy is one of four people out of 16 Disappeared whose bodies have never been found.
The others are Columba McVeigh, Joe Lynskey and Army Capt Robert Nairac.
The sister of one of the Disappeared, Seamus Ruddy, has arrived at the site of a dig for his remains in France.