1. After the demise of your grandfather, interstate, his property will be jointly owned by all his sons and daughters and also wife,
2. So, your grandmother inherited 1/6th of the property of her deceased husband,
3. She can gift and deal with her 1/6th share of the said property in any way she wants and not for the entire property of your deceased grandfather,
4. You can certainly challenge the said will,
5. Since the question of will shall come to fore after her death, it will be prudent on your part to file a partition suit claiming your share of your grandfather (through your father),
6. If your father is alive, he should file the said partition suit.