Mutation Issue

Dear Sir/Madam My Grandfather had registered a land on my father name as a gift in BIHAR. The registration was done in between year 1970-1980. Due to some reason my father did not do mutation of that land. After my father death we found the papers in his box. We showed papers to our advocate and on his green signal we applied for mutation and we got mutation done without any issue. Now my Uncle is saying that paper is forgery paper and mutation is not valid. He is saying if your registration was valid then why your father could not apply for mutation earlier why now after his death. Looks like he has given a petition or filed a case in "The Court of Deputy Collector Land Reforms". He is saying mutation is illegal as C.O. did not send him notice before mutation and without background check of registry papers he muted the land to my elder brother. My uncle lives-in some other place and not in native village, I think notice was sent to native place (I am not sure about it). Also on same registry paper there are other lands which grandfather has given to my father as a gift. Mutation of those lands have not been done yet as we thought we will go for mutation one by one but unfortunately uncle filed a case and now not sure what is going to happen. Want to know what will happen now? Will mutation be cancelled? Or further mutation will not be done unless case is finalised? In preparation against my uncle petition what we should do? Everyone involved in registry have died (Grandfather, Father, all witness). How can we prove that paper is not a forgery paper? I appreciate and respect your valuable times. Thank you in advance.