This implies that the emperor may not be heir to the throne. Or, may not be 'high born'. This was 2000-2500 years ago, perhaps.
Legitimacy came from the male in these times.
So, having a shared father means nothing bad for the emperor. Having a shared mother means his title, his lands, his bloodline may be entirely false, and thus, his legitimacy.
(Even if the emperor rose and claimed his place by blood/conflict, the above all still counts in many ways.)
This implies that the emperor may not be heir to the throne. Or, may not be 'high born'. This was 2000-2500 years ago, perhaps.
Legitimacy came from the male in these times.
So, having a shared father means nothing bad for the emperor. Having a shared mother means his title, his lands, his bloodline may be entirely false, and thus, his legitimacy.
(Even if the emperor rose and claimed his place by blood/conflict, the above all still counts in many ways.)