I read a spoiler months ago of the novel and if remember correctly, she wants to keep Theo the real crown prince “hidden” until the time is right to get rid of him and put her own son on the throne. I believe her son it’s not of royal blood so doesn’t have any right to the throne until they get rid of Theo late enough to not have any other option than to put her son in place. Or something like that.
Iirc her children do have royal blood but they are not in line of sucession since the former emperor made a decree that only the line of the first empress is recognized. Hence why the (2nd) empress dowager's children are on a grand duchal house (nobles with royal blood.) So basically they would have had the right to sucession if the former emperor didn't make that decree

i have no clue as to what the empresses motives are bro
also can we stop referring to her as a hag it feels a little misogynistic