Nomination does not bestow ownership to property. Ownership to title of the property would be governed by the law of instestate succession depending on your religion.
If the property is not partitioned prior to 2006 and is still in joint family name, then even the married daughters will have a claim in such property and any transaction in connection with the property will have to be done by all the legal heirs together.
If your father's sister refuses to sign the trasnfer deed in favour of your mother, then you would have to file a suit claiming title to the proeprty on the ground that your father had alone paid for it and the name of your grand mother was added in the title deed merely for name sake.