1.The proeprty belonged to your grandmother is cosndiered to be her self acquired proeprty and hence she is free to dispose of it in any manner she likes.
2. During her lifetime she can trasnfer it by way of a gift deed and after her death she can bequeath it by way of a Will.
3. As long as she is alive you cannot claim any share through your mother.
4.only if she does without Will Then only your mother will be entitled to 1.3rd sahre.