Why did he faint at times?

I really liked the series but there were a lot of details that they avoid at the end...  //maybe **SPOILER ALERT** // I just could understand that he may have any problem due to the way he born (?) yet neiter the doctor check on the symptons so Im still wondering. I hope someone else can tell us about.

Or epilepsy 

 levick58:

Was the king really expected to marry his sister? 

Yes, apparently that was the custom at the time so that the bloodline would remain "pure"

Low blood pressure

Anemia

Diabetic

I'm still thinking about that to this day. Also the princess seemed to have asthma or panic disorder.