I don’t think I have ever even considered this as a question, especially before I read the details. Obviously, you have won the dog’s trust. Is your sister a little rough or not as gentle with her? Your sister needs to look at why the dog trusts you more and maybe change a few of her behaviors. You can’t be rough or mean to the dog, that might seriously mess with her spirit. I had a dog I suspect was abused and it took a lot of work, but she came around. Your sister needs to work at it more.