From Longman Dictionary of Contemporary Englishmaternal grandfather/aunt etcmaternal grandfather/aunt etcFAMILYyour mother’s father, sister etc → maternal
Examples from the Corpus
maternal grandfather/aunt etc• Her background was middle-class, cultured, my maternal grandfather being a rabbi.• We are to christen this child Pilade, being the name of Ferdinando's maternal grandfather whom he wishes to honour.• He inherited the honorary title from his maternal grandfather, William Harold Pearson. 