Mental/Psychological harassment by sister's in law

My Christian husband and I have been constantly harass psychologically by his two married sisters with the support of his mother. Before I got married my husband's elder sister constructed a one storey building at her father's home,at that time she was already married and staying somewhere in a rented apartment with her first husband. Now that I'm married to his brother she constantly claims her right on that building and tells us through phone or through my mother in law that my husband and I have no right on that house and my mother in law also supports her daughter. Nowadays she is married to a Pakistani muslim and resides in Kuwait, she hasn't even divorce her first husband. She recently came with her 1year old kid and started claiming her right in that house. Few years back she even filed a false case against us which got resolved.Whenever we try to take step against it she along with my mother in law threaten us to put us behind bars. We are from a humble background. Please help