What a terrible situation to be in, man. I'm truly happy that you walked out and even happier that I've never had to experience something like that, despite what I wrote in this post. None of my siblings is actually selfish or conniving, but even the best person can screw up.
I'm actively ensuring that the dog finally gets some respite. The next step is for my sister to realize that he's got to go out regularly. I'll see how that goes. Thank you for your wonderful comment!