Prince Dal is the son of King Stefen, one of Randor's siblings.
Like Jeremy he is one of the cousins of Adam and Adora.
It is unclear if Dal and Jeremy are cousins though, as it is unknown whether Jeremy's parent is the siblings of Randor or Marlena.
However since Marlena came from Earth and no family is known to have come with her, it's pretty likely they are cousins and that one of Jeremy's parents is also Randor's sibling, like Stefen.