According to the pedigree of Anlawdd that has been postulated by T.W. Rolleston, Custennin was the son of Anlawdd and brother of Yspaddaden (Ysbadadden) and Goleuddydd.
The same author prepared a genealogy for King Arthur in which Custennin appars to be a simple variant of Kustennin.
See also
Constantine III | The Legend of King Arthur
Kustenhin | The Legend of King Arthur
Mustennin | The Legend of King Arthur
