Sister emotionally abusing parents what to do legally

Hi My sister has been secretly having an affair with a boy of different caste/social background without any of our knowledge since last four years. All the while asking my parents to look for prospective grooms in our community. And rejecting them on one reason or other. Recently her affair came to light and we told her it's unacceptable to us but she is free to do as she likes. Since then she has been emotionally and psychologically abusing my parents to accept that guy and marry her with all pomp and glory. That boy also bragged into our house unannounced to for us. He even sent me a abusive mail for advicing my sister against this relationship. My parents are old and have health issues. She and that boy are causing too much tensions. Is there any legal action we can take against them to stop their harassment! We just want them to leave us alone.