Does moving someone into your home affect council tax?

this is my mother and fathers house and they live with my sister, she has moved her boyfriend into there house and all his worldly possessions, it is a mortgaged property and my mother and father receive benefits for my 2 other brothers who live there and are mentally retarded. I don't want them to get into any trouble with either benefit fraud or council tax fraud, is there anything to worry about, also there is a woman over the road from them who will grass them up if she gets the chance so I want to make sure I don't have to help them out of any sticky situations in the future. Thanks

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