Well, there's not much you can do I guess if your sister isn't willing to give him up. You certainly tried and I must say, her responses to you are kind of aggressive and a bit mean. Afterall, you are just trying to help an animal have the best possible life.
I hope that Gus will heal up okay. It seems like cats who are adults who have their claws removed deal with a great deal of discomfort. I know of two declawed cats and the one who was under a year when her claws were removed does fairly well but the one that was 3 or 4 years old at the time walks as if he is walking on glass. It's very hard to watch. But, maybe if he can have the laser surgery with glue instead of sutures, it might help a bit? Not sure. Any chance of catnapping Gus???