Sister in law asking for divorce on grounds of adultery

This is regarding my sister's marriage. We belong to a Hindu Middle Class Family and live in Moradabad (UP). My sister got married last year Dec-2016. His husband used to work in Delhi-NCR. But suddenly in the month of Nov-2016, he got transferred to Mumbai. Since then, my sister is living in her Mayka (Not with in-laws). One day, her husband called up my father and started saying bullshit about my sister that she is having an affair even after marriage. He did not took my sister along with him to Mumbai and every time made an excuse. He claimed that he had pictures that shows that my sister has an affair. As per my sister, she does not have any affair with anybody neither had any in past. In April 2nd, her husband came to our home tried showing pics to my parents. Yesterday only, my parents visited my sister's in-laws house.