Guidelines to change the property from diseased father to me and mom

we are one son and one elder sister of my father. father buy a land which written a stamp paper with 5 witness. my sister is married. the land cost was 70,000/- where 50,000/- from my side and 20,000/- from my mother. before my fathers death he again wrote a stamp paper with 4 witness where he mention about my and my mothers donation.but all of 4 witness are dead. here only we have a jela parisad land licence of my fathers name. from the first i am living with my parents. now my father is dead and my mother want to change the licence of my mother and my name because my sister did not help any thing and rarely come.after my fathers death only i mentain our house and take care of my mom. plz help us.