I'm sorry, Catkins. I hope the blood tests bring good news and that your pup gets to feeling better. To my understanding there are several things you can do to help support kidney function; I had a Sheltie who had bad kidneys for a couple of years before he finally passed at a very old age. So all may not be lost, but I know how hard it is when we are worrying about our pups.

As to your sister's dog, I don't know what sort of behaviors he displays but my dog has a lot of behavioral challenges (and has had since he was a puppy, they are largely genetic/trauma based) and it can be really taxing. Training can help a lot too, but especially in my boy's case we've made a lot of progress but there will always be a high level of management too because it's just who he is. I know you wish you could help them, but I do think you are right in putting your girl first and there is nothing wrong with realizing something wouldn't be a good fit/would be more than you would want to take on.