If a boy and a girl are raised together by their maternal grandmother, this does not make them Mahrams to each other; they are cousins, and so it is lawful for them to marry each other, unless it is proven that their grandmother, or any other woman, had breast-fed them (Part No. 17; Page No. 398) five times or more during the first two years of their lives. This is the only case that would make them Mahrams to each other. In this case, the boy is the girl's foster brother. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.