It sounds like Chester should definitely remain at home and if your SIL can stay there, all the better; or at least perhaps stop by a few times a day? I'd write down all the things to look for re: Chester's pancreatitis and the vet's info. should she need to contact her/him. And I'm sure you'll be checking in often, too! As for Millie and Aneko, I'm not sure if Chester wouldn't be more stressed with them at the vet's???...but then you have Millie's aversion to strangers to consider, too! Maybe you could have your SIL drop by frequently, a few times a week, (maybe feeding them) prior to your trip to get Millie more used to her being in the house?