For Rhivenian Monarchs marriages with commoners are seen as second place and as far as I understand take place to compliment noble marriages,like that King Charles can justify marrying Queen Isabelle who comes from a common family perphaps even being from outside Rhivenia(was from Polaris,if that is another Kingdom or a Rhivenian territory no idea) thanks to his other wife Queen Alvena coming from an important noble background.
Also I presume that polygamous marriages are reserved only to the King of Rhivenia because the Everans twins father only had a wife their mother,if this is different on other families and a polygamous society is truly in place for the nobility is an unknown still,if in book 2 we do get to meet more nobles on Rhivenia this matter can be solved.
If general nobles/Princes can only marry once,Francis marriage in secret to Aldreda the Commoner without telling anyone on his family is all the more impactful,beyond the mistrust King Charles felt for his brother marrying the lass(if Aldreda did cheer on Francis to rebel doesn’t matter,his cause was doomed from the start,Aldreda came from nowhere and made Francis no favour with the nobles) he also lost an important bargaining chip,marking the marriage as an overall poor affair and a defiance even if it wasn’t an ambitious move by any means he didn’t ask for Charles’ permission and hided it as the King’s only remaining sibling.
Switching to Pagonia,Does that mean that marriages with commoners are more overlooked there?Nutella confirmed that Pagonia’s religion,Ateshios the God of Fire,allows only Monogamous marriage so no double play with noble and common unions on the same generation.
Perphaps we can also guess that illegitimate kiddos are also seen on a better light on the south,Francis marriage made on such a secretive way can also be debated was it official and trustworthy by Namherys’ rites?If not recognised by the Royal family did it count at all?
Nevertheless Astryn’s parents were smart welcoming pregnant Aldreda(if they knew about the parenthood or not do not know if it was implied or merely out of pure kindness,if they didn’t they were really lucky),as welcoming dissidents and using them against your rivals was a classic practice on feudal politics.
Like sponsoring Erwan’s against Rhivenia probably always has been a thing cause giving Rhivenia have forever been their rivals,Erwan was just their cart to play against the North for that generation,Pagonians apparently love a good clash against their neighbours on a way like the Scots did with the English.
Their many differences religiously(Ateshios vs Namherys,they call the later the Lord of Evil,Rhivenia’s religion is practically demonised by Pagonians perphaps Rhivenians do the same),culturally(Monogamy vs Polygamy) and personal political feuds(Astryn’s dynasty,Pagonians nobles vs their northern counterparts, Rivalry over past wars,murders etc) play an evident gateway to the whole conflict.
If welcoming Aldreda was Astryn’s parents master plan for the generation, they sure didn’t count with Astryn giving away their Kingdom on her absurdity cause 'Erwan is a good friend and deserves Pagonia plus his revenge".