Do realize japanese sucked at communicate and always keep to themselves right? It even potray it in every manga in every genre. Its stupid yes and at the same time it is extremely stupid for the mc and every mc in every manga are like that because of their "Oh look i don't want to bother someone...
That's.... entirely depends on the brother though....
Especially if the princess has carried a baby from a different world then um... There will be a dimensional crisis.
Well if you look at her perspective. Remove the prospect of FML father having an affair. But instead hating the idea of concubines despite the fact in medieval history it needs to have concubines to continue the bloodline