Hey Johnn, you can do DNA testing for dogs quite cheaply these days. I think they are down to 30 bucks on ebay. Some companies are better than others or have a wider range of breeds. It just takes a simple cheek swap. We used one to verify that our ACD was not a mix, because we want to keep with cattle dogs with her build, and had to ensure we weren't hunting after a mix.
my guess why they would think he is a mix is his "split" in his head(not sure if thats what its officially called) your pups is quite intense/deep but no i dont think hes a mix either quite a handsome fellow
I can kind of see what your sister was saying - his head looks quite broad for a JRT compared to his body. But then again, JRT is quite a varied breed so could be a normal variation.
My dog Jimmy is supposedly 'pure' JRT, but I think someone was telling pork-pies...
They are wonderfully characterful dogs though - never a dull moment! When my cockerel escaped he just sat and watched as I tried to catch the chicken.