Especially since her mother is likely a relative (most likely his legal sister given the reasons following the *) of his so she should be a royal herself. And what about her real father? He probably married the relative so why is he not sitting with them either? Another question I have is if they are actually all siblings, like did the same woman give birth to all of them? And yet another question is, in either eventuality, why did the woman who gave birth to her give her daughter up? *Unless the reason Octavia was given up was precisely so she could give birth to babies for her brothers. Ew.
I mean it's obvious that women aren't seen as potential heirs in this world either. So her younger brother is obviously the spare not her.
And if they all had different mothers they all likely had different fathers (most likely all married to their mothers), too. So where are all of their real parents?

I had the hope that the king would somehow step in and be on the mcs side on how the crown princes needs to think about the while heir thing before getting it on with his boy toy but no no hes in on it as well with a husband this is truly terrifying this means that the woman who gave birth to the kids dosent even qualify to sit with them during dinner this is too much no wonder the mc is doing all she can to not become such a disposable pawn (like her mother or any other rotal womam)