You're raising a great point. It depends on what the houses want and the power dynamic between them. Traditionally, the woman would marry into the man's house and then her brother/cousin would take over, but if her house is just as strong or maybe stronger than the man's, then either the man marries into her house, the houses merge or they each rule their own houses, but then they do have to have two children to give them each one house. If the woman's family is gonna be ruled by her uncle/cousin/male distant relative, then that one becomes the main lineage and then her and her husband's kids rule the husband's house only.
"there couldn't be another marriage between them". I think one generation would be enough for them to then have cousins get married to each other. The siblings would rule now, then each have kids and their kids could get married with no issues. If the author wants to be historically accurate, cousin incest is fine.
Lol I was trying to avoid saying it but the cousin incest isn’t too far out of reach this is why I like when at least one of them is a commoner/low level aristocrat, no headaches on this matter
Something I always think about in historical manhwa when the MC and the ML are both from ducal houses is how the power structure doesn’t crumble.. especially when they don’t have any siblings. Do they end up merging? Do they (somehow) each manage their own houses, then try for two kids and have them continue both houses? Then there couldn’t be another marriage between the two houses, at least for a while.
It’s different when it’s imperial family x ducal house since it’s not uncommon that a duke has some imperial lineage. It feels different when there’s multiple dukes who are world-built to keep each other in check, feels unstable