Property issues in famly please advise which is ancesteral

Sir/mam.. I have a question. My grand father has a property and lands which he got from his parents. My grand father has 2 daughters(my mother and her sister).my grand father is with us and my mom and dad are looking after him in his old age. My grand father wants to give little more share in property to my mom n dad as he with them. My grand father has written registered deed for few of his properties to my dad n mom. Now my mothers sis and her son is unhappy as the property is not equally distributed. 1) my questions for some properties we have regestered deed from my grandfather. Now will my cousin (mother sisters son) has any right to disturb the registered will after my grand father goesoff? How do we safe gaurd the property which he wants us to take. ? I heard they have one third share in all his properties is that true? What do u suggest us. Please advise