False case against daughter in law

,my sister came to know about her husband affair with another lady 6 months ago. She has all the proofs of it. But she doesn't want to go to police right away n is trying to convince her husband. But he is not ready to stay with her n want to marry that girl. His parents are also supporting him.he has taken 4lakhs from our mom when they were in good terms.now he is refusing to return it stating there is no proof of it. He is also abusing her physically verbally n financially.now he n his parents r threatening to file a false case of harassment against my sister.she is worried about her n her daughter future n financial status.please advise how to teach them a lesson legally...It's 4 yrs of marriage n it's a love marriage.