Mordack

Widely considered the most powerful villain in the official King's Quest series, Mordack was a good friend and "Brother of Society of the Black Cloak" to the former Vizier of the Green Isles, Abdul Alhazred, and through his relationship with Alhazred he was able to meet Cassima, princess of the Green Isles. He seemingly expressed his wishes to marry Cassima, and when she refused, he kidnapped her and brought her to his island and put her to work as a scullery girl. Afterwards, he worked a magic spell on the Castle Daventry which minimized it to a small size and he imprisoned it and it's inhabitants inside a glass bottle inside his laboratory in his castle in retaliation for the transformation of his brother Manannan into a cat by Prince Alexander of Daventry.

Mordack was killed shortly thereafter in a magical battle of iconomancy with King Graham of Daventry, when Mordack transformed into fire, and Graham into a cloud.

He is the brother of Manannan and Hagatha.

Mordack in the Unofficial Games
Manannan visits Mordack on his Island in the unofficial King's Quest III (IA) to discuss his plans for Alexander.

In KQ2 (AGDI) he is said to be also the younger brother of Manannan and Hagatha. However his relationship with Hagatha is strongly implied to be a connection to the Black Cloak Society, not a family relationship.

In the ZZT games, Ravenlos is said to be his cousin.

Real World
In the official King's Quest Collection trivia program, and King's Quest Companion, Hagatha is said to be his sister. Though this fact is not brought up in any of the manuals or the official adventure games themselves. Also, in an interview Roberta Williams stated, "Nope, Hagatha was her own seperate person". So the connection was made only by other official designers, authors or producers that have contributed to the King's Quest universe and lore.