Im happy im not your mom!I disagree with a lot of that. This is a matter of a mom not showing respect for her grown adult daughter.
If that were my dog, I would have flipped out and severed ties with her for a while. Dogs are supposed to bark at new people. I have two dogs, a rott/beagle that only barks at shady people, and an American bull dog that barks at everyone who enters HIS house if he doesn't know them. The rott/beagle has an amazing sense for picking out scum bags, and the bull dog not so much, so he does his job with everyone he doesn't know. Dogs are supposed to be protective of the people who take care of them, as well as their homes. If they didn't bark, I'd consider them defective, and useless for that purpose. To have a dog put down for barking at, or jumping at a new guy who walks into it's house behind your sister is absolutely INSANE.
As far as her concern with snakes, tell her to know what she's talking about before she opens her mouth. I would have gotten the yelling out of the way a LONG time ago, putting my foot down with that sort of nonsense way before it came to the possibility of her thinking it was okay for her to kill your dog. If you don't want to yell, at least write her a serious and long letter. It sounds like you have a lot you could say to her about this and probably a ton of other things she does. You're 27. It's time mom is brought down a peg or two. Even if she was a brilliant woman with infinite wisdom, you are still too old to be putting up with that kind of shadow, but she doesn't sound like a genius, so it's definitely time to back her up 50 paces before drawing a line for her to stay behind. Do you want to be like Principal Skinner your whole life, just so you can avoid ever hurting her feelings, which will heal?
Mike.