Leighanna Needham remains hopeful despite detectives finding a toy car they think was with her brother when he disappeared on Kos.
Ben's sister was speaking after South Yorkshire Police formally ended a three-week search on the Greek island, saying they believe he died as a result of an accident involving a digger on July 24 1991.
Although extensive searches of two sites on Kos did not find Ben's body, officers discovered what is understood to have been a yellow toy police car where a digger driver working outside the farmhouse where the 21-month-old was playing 25 years ago deposited earth and rubble.
Speaking to Good Morning Britain, Miss Needham said her grandparents were 90% sure the toy was Ben's.
She said: "But what I can't stress enough is that my nan is only 90% sure that it's one that's similar to the one that Ben had. We can't say for definite that it is Ben's toy car.
"Over the three weeks of the dig, they have come across a number of toys; that farmhouse has been a family home to many people throughout many generations so it could be Ben's, it could not be Ben's."
She said: "I still remain hopeful that he's out there".
Miss Needham said the family had been given the option of gathering at the farmhouse to mark Ben's death.
But she said: "That's not something we can do because there's no proof that he's there.
"So how can we say goodbye to someone knowing that there is still that hope?"
Miss Needham added: "I've got a pretty big heart and I take a few more because I want to find out what happened for my mum and for the rest of my family because they can't take any more."
Ben's grandmother, Christine, told the Daily Mirror she remembered buying him two toy cars in Kos town centre.
The 64-year-old said: "I was shaken by it and both me and Kerry (Ben's mother) cried. I just felt sick really. It was a shock.
"I expected it to have no paint on but I was 90% sure it was Ben's. When I saw it I felt disbelief."
Referring to digger driver Konstantinos Barkas, also known as Dino, she said: "It must have been dumped at that site by Dino. It didn't walk there."
Mr Barkas is believed to have died from stomach cancer last year.
Speaking at the scene on Monday, Detective Inspector Jon Cousins said the recovery of the toy added to his belief that material had been removed from the farmhouse on or shortly after the Sheffield toddler disappeared.
He said: "My team and I know that machinery, including a large digger, was used to clear an area of land on 24 July 1991, behind the farmhouse that was being renovated by the Needhams. It is my professional belief that BenNeedham died as a result of an accident near to the farmhouse in Iraklis where he was last seen playing.
"The events leading up to and following that incident have been explored by my team of experts to great lengths. The fact that we have not had a direct result during this visit to Kos does not preclude the facts that we know to be true.
"An item found on Saturday, which I have shown personally to some of Ben's family, was found in one of the targeted areas at the second site, very close to a dated item from 1991.
"It is our initial understanding that this item was in Ben's possession around the time he went missing."
Despite this stage of the investigation coming to an end, the officer pledged to continue searching for answers, and paid tribute to Ben's family.