Genealogy
In the Mabinogion, Dylan's mother, Arianrhod, is the daughter of Dôn and the sister of Gwydion and Gilfaethwy. Her Uncle, Math ap Mathonwy, is the King of Gwynedd, and during the course of the story, Arianrhod gives birth to her two sons; Dylan ail Don and Lleu Llaw Gyffes through magical means.
In the Welsh Triads, we are given a context for an actual Arianrhod who appears as the daughter of Beli Mawr and the sister of Caswallawn (the historical Cassivellaunus). Whether this reference is the result of the merging of a myth and history is unclear. It is possible that a later historic Arianrhod has become identified and merged with an earlier legendary/mythological Arianrhod.
Read more about this topic: Dylan Ail Don