I agree, that level of cleanliness is not really possible with a dog but especially a puppy, and leaving the puppy crated alone in the garage is unfair. Crating in the house while you're gone is one thing but the typical expectation for a puppy's bladder is, I believe, about an hour for each month they've been alive (only puppies, not for adult dogs) so a 4 month old puppy would be roughly 4 hours (someone please correct me if I have that wrong). I think everyone would be happier if your sister could find someone else to watch her sweet pup or as Betty said, if you could watch the dog at her house. You know what your lifestyle and personality are, and they don't really fit with having a dog even temporarily. It's good that you are aware of this!