1) did your grand father execute gift deed ? was it stamped and registered?
2) if so you are absolute owner of 5 acres of land
3) if any notice is issued to you contact a lawyer and reply to notice
4) if no gift deed was executed nor will made on your grand father demise if there were 6 sons each son would inherit 5 acres each . your father share of 5 acres would devlove on your mother , you and your siblings