Can my grandmother has rights to wrote a will?

Hi sir, I am from tamilnadu and i am also a christian .i had a doubt that my grandmother wrote a will that is,after her lifetime her property belongs to my father and for me (note:not after my father ,for both).the property was ancestoral property .so my doubt is whether she has right to wrote a will.i have also one sister .does she can claim her right on this property .one thing few years before my father sell a part from this property .in that selling document my sister name was included and mentioned as (she was my grand father's grand daughter so she was added in here) .so tell me she has share or not.if she has ,does she has equal share or may be1/4 like that.or according to the will the whole property belongs to me .we are going to sell this property .so please suggest what can i do legally .some says my grandmother can write .some said she cant because .according to the will she gave her property to me and my father .so i have 1/4 .but after my fathers death .that fathers share my sister and i have equal share .so i have few more than my sister is that correct or not.or