The brother become obsessed with his sister right? or it just me?

yes, also their not real siblings
the brother is from another man
cayena is the only one with royal blood
crown prince and the ml are half brothers

So they are siblings

I need to go find the thread again but as I can recall they are don't siblings XD

Cayena and Rezef have the same mother and different fathers.
Rezef and Rafael have the same father and different mothers.
guess I was wrong ohh well, half siblings then
sorry for the wrong info earlier