Gee, it really does sound as though Chester would be too stressed out if he takes the trip, especially as he becomes agitated if left alone and you won't be able to spend muc time with him. And as Millie doesn't take kindly to strangers and your sister in law might not be able to detect the signs and symptoms of pancreatitis in Chester, I'd have to vote for boarding them all at the vets. Would Chester be stressed out if boarded there? I think you'd feel much better knowing that Chester is being carefully monitored by his vet where he would receive medical attention if needed. Perhaps Chester would be more comfortable at home with your SIL (and Millie and Aneko at the vet's) but I fear that if Chester did have an acute attack and your SIL did not pick it up, she would be devastated. This is a tough one!