
Well, that's just my suggestion, but I've got 2 ideas.
1. There's a different timeline between humans and demons. In that case it would be possible that Rian isn't her daughter but her granddaughter or something like that. Because we still don't know how her father looks like.
2. And I believe it's more possible. His sister was in love with the current emperor. Because I think her love looks a lot like the emperor and there is the open case with the broken promise on the side of the king.
I'm unsure if the first prince is her son or she really was the first queen, but I do believe she loved the young prince before he becomes emperor and was somehow betrayed, so that she died. ╥﹏╥

here ML uk our demon crown prince
we are discussing about cecilia(dunno if i spelled it correct,ML'S Sister ) being our fl's mom
so tohato thinks
it might be either
our fl is grand daughter of ML's sister
as demons live longer than humans
or
fl might not be related to MLs sis
but crown prince might be as his hair color is similar to the guy ML's sisters loved and emperor must have been the lover who betrayed ML's sister
is she their daughter ? (⊙…⊙ )