@Add1152000 and everyone else. At least in the webnovel (don't know about the LN that this is based off of) the sister that is shown is a half-sister, born from the first queen, (The current one is #2). The brother is the second prince (Haruto is the first born) and is 100% blood related.
For what happened to the first queen,
For what happened to the first queen,
The king's not entirely stupid, as time passed by and seeing the current queen's attitude, he suspects that the first queen was harmed/did something that caused the first queen to pass away. The current queen used her influence/popularity as a the hero who killed the demon king to become queen and use that same influence to slowly undermine the king.