I'm 100% certain that Gwyn is related to the Vanserra Family she's also the only other fae to use her family name like the Vanserras and she said her mother lived in The Forest House but anyway I think either Eris or Lucien is her grandfather. Lucien due to his relationship to Jesminda since we don't know what kind of faerie she was and it's quite possible but in the case of Eris i could see his redemption being more of familial love for his granddaughter instead of romantic as well as Cassian saying he's possibly a good male but he's to much of a coward to act on it. Also when Rhys mentioned the firstborn child and Eris said he doesn't know about the firstborn instead of my firstborn now this may be a stretch but its possible Gwyn's mother is Eris' firstborn who i feel he gave up to the priestesses to protect her from Beron since we know what he did to Jesminda and as well as what he does to his family. So to Beron, he'd view his granddaughter as an abomination that needs to be disposed of as well as to keep his bloodline pure so basically some Death Eater shit and i could see Eris trying to become High Lord so that he doesn't need to hide it anymore and so him and possibly his brothers can love who they want without their father in the way.