The king's chief queen, usually the mother of his heir, the Great Royal Wife was often the king's full or half-sister (the eldest daughter of the previous king and queen). Marriage to her enabled the king to inherit the throne, since descent passed through the female line. However, in some cases (for example, *Tiye, queen of *Amenophis III), the queen attained this status and position for other reasons.
Biographical Dictionary of Ancient Egypt by Rosalie and Antony E. David
Ancient Egypt. A Reference Guide. EdwART. 2011.