I really enjoyed this game, it's the first in a while that has me chomping at the bit for the next instalment.
While the premise is not original, it's very well executed, and as others have mentioned the cheating scene is not 'in your face', and serves as a sensible trigger for subsequent events.
I like the art style, it's well executed, although some more intermediate images alongside the text, would probably help the flow in longer conversations.
The slow pacing of the story works, and it appears to be well structured, with logical progression and scene changes.
Suggested tweaks and changes (some already suggested):
An explanation of the King/Prince choices and what they do.
A review of the English: some missing words, some typos. Also for the longer Spanish sentences and some phrases, an English translation will help improve immersion for players who don't know much or any Spanish.
You've added an option to rename the MC, it will be worth doing the same for the Love Interests too.
This is a refreshingly good incest game, keep going dev!