You should file in application under order 7 rule 11 for rejection of plaint as the said property is the self acquired property of your maternal grandfather and hence he cannot claim any share during the lifetime of his father. You should also audios documentary evidence to show that the the subject property is a self acquired property and not a ancestral property under any circumstances. If you are able to do that, his case would be dismissed.
Also, you cannot sell the property without taking the permission of the court as the order clearly states that if the subject property is sold permission of the court would be required and if you do so without taking prior permission, you will be held liable for contempt of court.