I imagine this must be scary for your sister as it seems to happen when she is alone with a big dog - too big for her to carry to the car if needed.

These symptoms sound serious to me, and the need for more skilled veterinary care is called for. I'm not sure an emergency vet would be able to help, I think sheis on the right track asking for a specialist. But which type of specialist? Maybe that is whythe vet is holding off, hoping that osmething in all the test results will indicate that.

And Ben must be terrified as his body is betraying him.

I hope she can find the proper medical advice to help! Best wishes for all.