[quote=@ineffable]
[@Thinslayer]
Woah wait, so why is my sister here?
Is the king my dad? 
What is going on?
*existential crisis mode*
[/quote]

According to your character application, Isabella has royal blood, meaning that at least one of her parents was either on the throne or married to it. Given that both her parents were non-royalty, it suggests that she is a bastard child, with a former concubine mother who married Isabella's step-father after the king divorced her mother. Her sister, therefore, is more of a step-sister.

And since Isabella has a potential claim to the throne, King Alexander Dragan likes to keep an eye on her.