In KQ2 (AGDI) he is said to be also the younger brother of Manannan and Hagatha. "Brother" in this case is ambiguous, as the members of the Black Cloak Society (unofficial) use the term as a title, whether or not there is blood connection. However, Manannan does call Hagatha his better "sibling", and its confirmed in KQ3R that they are in fact related through blood. Manannan's letter to Hagatha mentions that Mordack is awaiting Cassima's birth and planned to wait until the girl is of age before kidnapping her and forcing her into marriage.