Question: Who is Isabella Of Bourbon's paternal grandmother?
Answer: Marie, Duchess of Auvergne

Charles de Bourbon (1401 – 4 December 1456, Château de Moulins) was the oldest son of John I, Duke of Bourbon and Marie, Duchess of Auvergne. He was Count of Clermont-en-Beauvaisis from 1424, and Duke of Bourbon and Auvergne from 1434 to his death, although due to the imprisonment of his father after the Battle of Agincourt, he acquired control of the duchy more than eighteen years before his father's death. In 1425, Charles renewed his earlier betrothal by marrying Agnes of Burgundy (1407–1476)