Idk if this is correct but I think it has to do with solidifying their position and power. Since the only reason the current emperor and empress are in that position because MC’s mom didn’t want it. With MC’s mom being the next in line to succeed AND marrying someone who had so much political power
because without an heir, the next in line for the throne are the emperor's siblings, like the mc's mom. the nobles will pander to them because technically she's currently the "crown princess" and even when she gave up her right to succeed, she's still a princess and her children are her heirs making them still next in line.
the order of succession changes if the emperor and empress finally sires a direct heir. and of course the nobles will shift their attention back to the royal family since they'll be snatching the crown prince/princess title now.
Sorry, if this been asked already, but why does it matter that the empress and emperor are having a child, I’ve tried re-reading but i can’t find myself to care… so if someone could explain, I’ll greatly appreciate it, as it seems crucial to the plot…