Invasion of Privacy and defamation

Sir, Girl got engaged 3 months before her marriage by her parents. The engaged girl and her fiancee moved around together few places (like park , Restaurant etc) after their engagement. Also involved in private chats between them including their post honey moon plan (including intimacy). But day before her marriage, the boys parents told the boy to show his watsapp chat. The boy's (fiancee) parents read their private and intimate conversation... They invited the girl's father home saying "family get together".. Four of boy's family members joined together and defamed the girl's father and the upbringing and the character of girl etc... and even threatened that they will cancel the marriage and there by will cause money loss, which Girl's father already spent towards marriage arrangement, Kalyan mantap, food arrangements , flower decoration etc... Now how the Boys's family can be punished under Law? Invasion of privacy and trust, threatening , Defamation etc.. Article 21?