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.

MTR was one of four companies short-listed to bid for the contract late last year.
The other train companies bidding to run the franchise included National Express, Arriva and French transport firm Keolis, through the Go-Ahead Group.
MTR is expected to employ 1,100 staff.
The franchise will run for an initial eight years with an option to extend the franchise for another 10 years after that.
London mayor, Boris Johnson, said the award of the franchise represented the "final pieces of the jigsaw puzzle" being put in place.
"I am delighted that we can now announce the new operator of this landmark railway, bringing it one step closer to welcoming passengers aboard," he said.
"Crossrail will revolutionise east-west transit in the capital, making London an even more attractive place to visit and invest."
The rail franchise will employ around 400 drivers and there will be more than 50 apprenticeships for people from communities along the route.
MTR already operates the London Overground franchise with Arriva UK Trains, as well as running the metro system in Hong Kong.
The huge Crossrail project, costing around £15bn to build, is the first complete new underground line in London since the Victoria line was built in the late 1960s.
The line will see trains running from Reading in Berkshire to the west of the capital and as far east as Shenfield in Essex, as well as linking to south London.
It is also one of the most ambitious transport construction projects undertaken in London since the original underground was built over 150 years ago.
Crossrail is expected to reduce passenger overcrowding on underground trains in central London and to reduce cross-London journey times.
TfL said it expected Crossrail to support the equivalent of 55,000 full-time jobs around the UK.
New trains will begin entering service in 2017. The route through Canary Wharf, the City and the West End will open in late 2018, with the full route running from late 2019.
MTR will start running services from the end of May 2015 between Liverpool Street and Shenfield, when it takes over the existing stopping services currently operated by Abellio Greater Anglia.
London Underground managing director Mike Brown said the award of the franchise was "a huge step towards the new railway coming to London."
The sister of one of the Disappeared, Seamus Ruddy, has arrived at the site of a dig for his remains in France.
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The £1.4bn contract to run the new Crossrail service through London from 2018 has been awarded to Hong Kong-based MTR Corporation, Transport for London (TfL) has announced.