As per the post it appears that an order is already passed and you have gone on appeal . So prima facie there are no proper documents to establish your ownership.
The case is appearing to be difficult.
The core point is why the second land was not registered? Who is holding the original deeds related the land? When the case against you was filed? How many years elapsed by then?
The question why the land was not registered really baffles one. 'Forgot' cannot be so easily accepted. There could have been more reasons. Either the amount of price would not have been paid in full.
You should get the services of a good and experienced lawyer. Even for establishing your rights with circumstantial proofs, you need agreement, sale deed(even if unregistered), receipt for paying the consideration amount as mentioned in the sale agreement and sale deed. Then all these should be supported by possession certificate, tax paid receipt etc. It may not be so easy.
In case the other party is innocent of what happened, then try to convince them and arrive at some compromise. Get the help of some village elders who know the correct fact and convince the other party. It may need some money to be given as compensation. Do it if t can be mutually accepted. Now that the original owner(one who sold land to your grandfather) also is no more it is complicated.
Please take help of a good lawyer because the other party may even file case to evict you. Hence you may need an injunction to prevent that.
The best alternative is to arrive at a mutual agreement and consensus.