OK, maybe this is cruel for me to say...but there's really NOTHING you can do as long as the dog has a roof over it's head, food and water. That's it...period. I think all you'd be doing is causing a big riff between you and your sister.
I know you don't like the way she treats her animals, but there again they are her animals. Kinda like people and their kids. I don't like the way a lot of people treat their kids but there's not a thing I can do about it.

I hope one of these days you can make her realize she needs to do better by her pets. I know how aggravating it can be.
Lots of (((HUGS)))
Anna