Is unregistered family partition deed is valid?

Hi Sir, My father and his brothers(5) were involved in family busiiness and bought properties in each others names when they were together. They partitioned the properties on 1998.They had the details documented but not registered, totally 18 people signed that document. My father got a property which was in one of his brother's name. Since then we were using the property. My fahther got a loan on this property(in his Brother's name), We paid all the EMI. unfortunately my father died in a accident in 2006. Later, Me and my brothers got this property out of debts in 2008. Now My father's brother has selled this property to some third party without our knowledge. Still we are in possession of this property(We have rented it to 4 diffrent tenent and we are receiving the rents). The party who bought this property seems to be thugs with local police influence, who are constantly bargaining with us. They are threatening saying they would get us arressted for land cropping. I am to be married this NOV 2, so we didnt want complication till then. Please help us, how to handle this. what needs to be our first step? I am 25 yrs woman working in IT. I have 2 elder brothers.