ONLY PAYING MUNICIPAL TAX BUT HAVING NO HOUSE AND LAND DOCUMENTS

My father is paying tax(municipal house tax,water tax) for almost 20 years of a house and we do not have land patta and papers because the site belongs to my fathers son in law.The only thing we have is municipal tax receipts and electricity bill receipts in favor of my fathers name.but now we want to sell this property but we have no land related papers.How can we sell it? And recently after 20 years the owner of this site is claiming that this land belongs to him ? what should we do now?my father has become sick and the house was built by my father 20 years back and we are living in that house for all these years?Do adverse possession come into picture here? These sites are steel plant displaced sites?The person to whom this land allotted by steel plant(my fathers son in law) remained calm all these years and we had no issue with him till now. So how to get land papers and title in my fathers name? We have only the receipts of tax and electricity bills bearing my fathers name.