I think it's more of what her presence means that doesn't fully solidify the current emperor into the throne because the duchess will always be a thorn (in a way at least) either to replace him or is worthier to have it. Which then leads to factions and instability between the aristocrats and the emperor. Kinda what we're seeing rn with the emperor hating the grandfather. Plus she may still have the rights to take the throne. But thats on the assumption she didn't give them up when she married her hubby.
What lotus says + now that the emperor has a child, the duchess's children are a threat to the line of the throne. Assuming the duchess doesn't have any other sibling with children, before the queen's pregnancy her son would technically be the first in line to the throne
can someone explain the relevance of the mother being former royalty/relation to the throne? like why is it worded like she was or could be a threat to the current emperor?