You and your brother are lineal descendants of your grandfather.
Your dad is not a lineal descendant of your grandfather.
However, if your mother died before her father then on the death of her father if he also left an interest in his residence to her surviving spouse (ie your dad) then the residence nil rate band is available for offset against calculating the IHT on grandfather's estate with respect to the interest left in the property to your father.
In short, the property worth 325k is left to the three of you and thus qualifies for the 100k residence nil rate band.
Transferable nil rate band and transferable RNRB may also be available to grandfather (but may not be needed).
Thank you. You've made it very clear for me to understand. No, didnt remarry.
Many thanks for your help. Really appreciated.