Don't worry. The WILL need not be registered. Even an unregistered WILL is ok.
I think WILL is not required to be probated in Tamil Nadu. However, I am not sure about this. Pl. have this aspect checked up and verified from a Lawyer in Tamilnadu.
It will be enough if you can get the property mutated in Revenue Records based on the WILL. Probably, the Revenue Authorities may require NOC from the children of the deceased. If the NOC comes from the children, the matter will be very very easy. However, if the children refuse to give NOC , then you have to get the WILL probated by approach the appropriate Court.
Till such time the property gets mutated in the name of the Wife, please keep the Original WILL in a very very secure place. Even after mutation, the Original WILL is to be preserved and kept safe for all times to come.
If need be, use only photocopies of the WILL.