I think Angenas take the land in previous Angenas head(empress' father) time. Even Yovanes didn't know the details.
I mean she was already an empress and had a son(first prince). In my opinion if she didn't engage herself between Angenas-Lombardy cold war, or didn't engage much intrugue, Yovanes would have made the 1st prince the crown prince. Because Yovanes kept 2nd prince in palace to keep to 1st prince/Angenas in check. If empress and her son behaved good, Yovanes would not let 2nd prince in palace
And all the privileges would easily pass to them.
As for the lands Angenas lost—I don't recall all the exact details—they had borrow massive debts to other nobles and to Tia's company in an attempt to expand their influence and power. Yet, as an already influential family, they had no need for such measures.
Furthermore, regardless of whether the family lost land, the Emperor and Empress were already married; the loss of territory would not have significantly affected her status.
Because the Empress desired to be the sole, all-powerful figure in the kingdom, she made increasingly aggressive and reckless moves whenever she encountered an obstacle. Attempting to assassinate the Emperor immediately was her biggest mistake.
They lost a lot of money on the project they were working on. I think it was a vacation resort or something like that. That's why they borrowed money from Tia's company and from Sushou (I can't spell it correctly ). Remember when Perez bought all the wood/logs and then sold them to Angenas at a much higher price? They had no choice but to buy them just to finish the construction. In the end, the vacation resort didn't do well because Tia's cruise ship became more popular. Lol.

If Angenas had just behaved themselves and do nothing, Yovanes probably would have made the 1st prince Crown prince. But because of their greed they lost an empress, a prince and literally a kingdom DUH