@Binary10011 I believe one of the ministers was attempting to get someone from his own family on the throne so the emperor rejected his idea and instead decided to marry Xie Yan using the same rhetoric that the minister was trying to use, that it is god's will, only twisting it to fit his own needs
@twilightspirits The emperor already have sons, the matter of a heir is more about which one will have more backing, and possibly that they might have a custom that there must be an empress when a crown prince is being chosen.
@Binary10011 because she is an orphan child now. And judge by the look from the outer court, Xie family influence and allies are diminishing. Imperial harem is the smaller version of the outer court. With no backing for this young child, those wicked ones will come out in force to hunt her down because of her rank. That is why this is like a death sentence for a child as young as that.