It isn't a matter of fair or not fair. Your grandparents property is theirs to dispose of as they choose, while they are alive or after their deaths. People frequently make revisions to their wills, especially as time and circumstances change. Your father dying before his parents is unfortunate. No parent wants to see their child predecease them. If your grandparents had passed away first, your father, as one of their children, would have collected his share of their estate, his inheritance and done with it as he saw fit. Because this was not the case, your grandparents are dividing between you and your sibling what would have been left to your father.
This is legally referred to as "per stirpes" (which is Latin for "per branch") Per stirpes is a method for distributing the estate of a deceased individual. Per stirpes specifies that each branch of the deceased person's family receives an equal share of the estate, regardless of how many people are in that branch. Your mother (their daughter-in-law) is not any blood relation to them and she is not considered a direct descendant of their lineage.
After you inherit, you and your sibling can always choose to share your portion with your mother but there is no reason to expect your grandparents to give her anything. Their estate is following a line of descent, a family lineage or blood line. My parents have made the same provisions in their will, if the same circumstances were to occur. The grandchildren step into the place of the deceased parent to inherit. The spouse(s) of any children are not considered heirs.