If they made clear that would be a mess. It would be clear that the adobted mother is infertile, but the public don't know this, thye know that she given bith to ML and you can't just go to the Emperor like, hey you r@aped my adobted son's mother now here you're daughter who is resembling you so much that you can't even denie it.... Later after the blonde got throw out from the duchy, the imperial family going to find the blonde b**ch, and she is like copy made from her father, and the Emperor made her a princess. And here the trouble voming again. But in this way the public still know that the current ducches is the birthmother of ML, and the blonde was a commoner, so nobody know the real deal between them. I think first they don't wanna the public know that the duke sister was r@ped by the shitty emperor, like even if she is already deat (I rhink) the people still would badmouth her.and her brother who couldn't protect her(in that time the male relatives dutie is to protect the sisters until they got married off), and secondly because the opponent is the Emperor, who know what would he do if his sin came out the light? Maybe kill them off, frame the treason or who know what.
The ML and the blonde shl*t are blood related (half siblings, the mother is the same) , but she doesn't know it. Their mother was r@ped by the emperor and that’s how blonde shl*t borned - this revaled at the story end, at a public trial where the Emperor and the blonde shl*t got demoted to commoner. .
ML is adopted, he was the Duke’s nephew, but his mother abuses him serverly, this is why he agreed marry with FL, he noticed the signs of abuse.