I'm confused, this description is for Llyra and not Dorma.

Aaah, my mistake.  I thought you meant insects, as in bugs.  You mean incest as in sleeping with near relatives.  I don't know if there's incest in the Grey Summers families, there's too many of them, from too many timelines, sleeping with too many people for me to keep track -- but I don't recall any incidents, even the one you are referring to.  Is X-Man supposed to be Nate Grey?
 
As for Namor ... He's a royal, and royals marry royals -- which usually means cousins of some degree if alliances don't need to be made.  Just look at European history, or even Black Bolt and Medusa, who are first cousins.  Dorma was at least a fourth degree cousin, IIRC, as she recited part of her lineage at their wedding ceremony, and Tha-Korr (Namor's grandfather) wasn't mentioned in four generations.