Helga Moddansdóttir - Sources for her husband as Magnus Moddansson. Did she marry her brother? her Cousin? or is this a Mismerge?

Although we know she is Hakon's mistress -
"HELGA, daughter of MODDAN of Dalr in Caithness & his wife ---. Orkneyinga Saga names “Helga” daughter of “a farmer…Moddan…living in the Dales of Caithness” as “Earl Hakon´s mistress”[942]. Her stepson banished her and her sister Frakok Moddansdatter to the latter's estates in Helmsdale, Sutherland after the death of her son [1131]. [http://fmg.ac/Projects/MedLands/SCOTTISH%20NOBILITY.htm#_Toc268450047 (Cawley's Medlands)]
Does somebody have sources to make her Magnus `Orfi` Moddansson
wife too?
Being that HER father is Moddan, of Dalr, it seems likely that this is either:
-NOT her husband, but her brother;
-or that it is her husband, who is also her cousin
-or that it is her husband ,who has acquired her 'surname' through a merge.
Anyone know?