Good luck we had the same problem with our neighbours when we first got Abby she complained all the time and complained to the council I was lucky as they told her as it was a puppy she had to allow for it to settle in(obviously someone with a dog). Her dog starts barking at about 8 and carries on all day until about 8 at night everytime I go into my garden he is growling at me through the fence and I have never complained.
I eventually got so fed up of her complaining that I told her exactly what I thought of her and havent heard a word since.
Can your sister set up a camera and record exactly what the dog is up to when she is out if he isnt barking she will have the proof that this woman is just being an old bag.
Mary