I think I need to tell my sister to get rid of her dog
Is there something wrong with the dog?
It's nasty and I hate animals anyway.
Not liking animals is fine but you shouldn't tell your sister to get rid of her dog just because you don't like it. You can ask her to put the dog away when you come over.
I hate dogs. I don't trust dogs. The dog is not even my sister's.
Even if you don't like dogs that is not a reason to tell someone else to get rid of her dog. You can always meet at your house if you prefer.