Mother father harrasment

sir my parents are 82 & 76 yrs old .my father borrowing money educated my sister now she is a state government employee. Giving lots of dowry he marries her with a state government employee. Now after 22 yrs of marriage she harrassed my parents so much for one property which my father purchased in his own.Am doing my job 1500 km far from my parents so she is harrassing them so much. please sir guide me what my parents can do now any law is there please guide.thank you