Yep, responses pretty much predicted from people. Sigh, one last try.
OP - you're correct, I don't know her. Everyone on this thread doesn't know her - or know you, for that matter. Although I notice this hasn't been brought up in response to people who're agreeing with you.
I'm not sure why you posted in AIBU if you're going to get riled about an alternative view. Unless of course, you're just looking for validation for your course of actions - and you've certainly got that from pretty much everyone, so I don't see why one person with an different perspective is such a bad thing but it's interesting that it's singled me out as a target on the thread.
We've just got your word for how bad you believe her behaviour to be. But all your posts show more compassion for the dog than your sister.
Also, I can't help noticing that she appears to be younger than you, and she's either a half-sister or step-sister, with autism, who must've required a fair bit of care from her Mum, in addition to getting inheritance from grandparents which allows her to buy things you consider pointless. That must cause a bit of resentment.
In regards to the dog - I agree that it's likely a bad idea and I think your role in talking to her and advising her not to do this is absolutely appropriate and supportive coming from her sister. But again, you don't categorically know how she'll deal with any given situation.
Regardless - and I can't stress this strongly enough - it is not your job to police what your sister can and cannot do. She's an adult and we don't know if her behaviour is as you describe - from your posts, you don't seem to like her very much. You talk about your mother in much the same way - calling her gullible and naive. You are not your sister's carer, you are not her mother, you are not her manager, you're not her case worker; so strongly-worded advice is where you need to stop. If you're that worried about her living on her own, I'd have thought you might have spoken to Social Services to find a way to support her, instead of only intervening to phone up a breeder and telling him not to sell her a dog.
If she's got enough money, she'll just go to another breeder. And then another one. Are you really going to stalk her behaviour and then call every breeder she speaks to, to tell them about this? Why are you trying to impose your will on a situation that's out of your hands and out of your control?
I have a relative much like you, who knows me very well. She felt entitled to tell me what I could and couldn't do. Told me I shouldn't be living on my own, told a couple of my old boyfriends that I wasn't capable of empathy or loving them back but I was vulnerable and very easily fooled and that I would be a handful for them to cope with.
She even called Social Services when I was pregnant as she believed I wasn't capable of looking after a child and I'd emotionally damage it, or neglect it, or get bored with it. She was doing it "for my sake, and the sake of the baby." I ended up being interviewed, them doing a home check and seeing me a few times after the baby was born, then being completely discharged as they didn't see any risk and were happy I was fulfilling my duties. I was tempted to frame and gift her the SS letter saying I a good parent and provided a secure, happy and age-appropriate environment for my child.
It doesn't matter whether you agree with her actions or not. You have no legal say over her behaviour, so as much as you want to control this situation, it really isn't up to you to decide what she does or what she's capable of. I wonder if you might have a misconception that her life is so much easier than yours and you resenting her trying to get in on "your" thing. When, actually, she might just be trying to find a way to identify and relate to you, in the mistaken belief it'll give you both something in common.
Leave it to the people who are responsible for looking after your sister and, if you're worried, raise it with them. But beyond that point, you have to walk away and let it go because you legally cannot do anything else.
I think everyone's missing the point on this thread. It doesn't matter if we think it's a bad idea and could be potentially dangerous. We don't know the full situation, we don't know the relationship and it appears that everyone's advocating for one sister to be able to control another and insulting her to strangers, without being able to verify the context.
But ok, whatever - ASD's the worst thing, we're utterly incapable of caring for other living things, we make everyone's lives terrible, it's such a bad idea for her to have a dog, everyone's just concerned for her safety and none of this is patronising, prejudiced or authoritarian whatsoever.
That magical term "autism" seems to makes it ok on here to justify writing someone off as incapable and "needing" to be controlled.