The facts posted suggest that you are in a hurry. The facts are simple. It is your haste that has made the issue complex.
On the death of your grandfather, the property has devolved upon your mother and the grandmother in equal shares. The grandmother having made the Will, you are the holder of her share in the property. (But for the Will your mother would have been the sole owner).
You can dispose the grandmother's share, which is now vested with you, at your discretion.
As regards to share held by your mother, you or or your sister have no rights or interest therein. You are only the legal heir and as such, could only inherit any property that is left intestate.