Address it to the agency or employer of the husband if nakaalis na siya pero ngayon habang andyan pa siya, pwede iaddress kung saan nakatira yung husband niya. But just an opinion don't file a case yet try to settle things, I could make your sister and her husband relationship shatter, early pa naman ehh, it could also clog the dockets of the court specially kung nasa city kayo. As much as possible save the marriage, baka nabubulag lang yang husband nya. Nevertheless, kung makikpaghiwalay legally yung asawa niya, hindi grounds ang sexual infidelity to annul the marriage. but if legal separation (this could only be filed by the innocent spouse) yung sister mo lang pwede magfile, they could live separetly from bed and board, but the marriage is not dissolved(bawal pa rin sila magpakasal), the property regime is dissolved also and liquidated and the guilty spouse cannot inherit or claim any in the absolute community or conjugal property.