Eileen Mitchell said her sister and their parents had been Christmas shopping when Katie spotted a Thomas the Tank Engine DVD. She was unable to buy it as the store had just closed its tills.
"She couldn't understand what was going on and my mum pleaded with them if she could pay the £2," Ms Mitchell said.
"Mum even offered to come back the next day with the DVD to see if they could take a photo of the DVD so they would have the barcode," she added.
Ms Mitchell said her mother tried to explain that Katie has a learning difficulty but staff became "quite abusive with my parents. They started swearing at my sister."
"She was starting to cry" and "my sister was told to get the F out of the shop," said Ms Mitchell.
"One of the staff members asked my mum if Katie had a leash, if my mum would put her on a leash to take her out of the store."