Just search all the places where your father in law would have kept the documents safe. Make sure that he has not pledged it to any bank or to some individuals for some purpose. One that is ensured you may do the following steps.
1. Dig into old papers in your custody or those which your father-in-law was keeping. Find tax paid receipt of any past year preferably a recent one.
2. With that tax paid receipt go to the village office. Ascertain how much tax arrears is there. Pay that and get the latest tax paid receipt.
3. Verify the tax paid receipt and compare the sale dd number in your old receipt and new receipt. If there is any difference you can sense some issue.
4. With those details go to the registrar's office(SRO) and apply for a NIL encumbrance certificate on the land.
5. Ask for a certified copy of the related sale deed.
6.That document will tell you the details.
7.If everything is okay, you can think of changing the land in mother-in-law's name.
8. If all the legal heirs join and give a relinquishing deed n favour of your mother-in-law, that itself will become the deed for change of ownership. Register that deed and then get the ownership changed in village records.
All these steps are possible only if the land is still in the name of your father-in-law.
Also please note that there is nothing like 'duplicate documents. Either it is original document or any number of certified documents. Creating further transactions without original documents is always risky unless we are amply sure that the original is lost for ever and is not in pledge or in some litigation.
In case the land is not in the name of your father-in-law, you will know it when you ask for paying taxes. Then you may have to get the deed number,but the officials may not give the details to you. You have to convince them somehow and get the details. Then get a certified copy of the deed from SRO. Consult a lawyer to know how these happened. If there is any kind of cheating involved, then you may have to go for criminal case.
If the relatives have some documents and they have not registered the land in their name , then I presume that it may be something related to giving security for money borrowed. So they may be having the original documents and some documents giving the land as security. I that case they may claim the land or money from you to return the land. The truth will come when you approach things legally.