Has your sister or anyone in the family been to Dorset England?
The reason I ask is these are relatively common around Dorchester in Dorset thanks to a huge hoard found there in 1934, many of which were Gordian Ants and Denarii. About 30000 made it as far as the museum, but many more found their way into builder's pockets and for a while you could get a pint of beer in Dorchester for one of these!
They still turn up regularly at antiques fairs and car boot sales in the area so someone may have picked it up as a souvenir.
Paddy