My sister is being accused of being the mother-in-law from hell. Advice?

She was protecting her daughter from mental and verbal abuse cancer patient. The daughters husband was verbally abusive and telling her to die and that their child was afraid of her and he was even bringing his mistress in the home in front of wife and mom he then said everyone was lying and picking on him he broke his wife heart and was so cruel and uncaring now mother in law has to be supervised to see grandchild the daughter passed away he told home health care that mother in law was from hell and caused problems she is so devestated and can only see granddaughter at daycare many people witnessed him abusing verbaly the wife many are nurses how do we prove this to home health care