Spoilers for the latest chapter:
Fjord is one of many children that the Duchess experimented on to bring those imperial magic powers (whatever those are called) back.
Fjord >probably< just turned out to be the most okay and/or powerful of them which is why he gets to be the "official" out-in-public son.
Lisette >might< be him, but with the info we have now it could also be another test subject.
Who knows what kind of fucked up magic the Duchess used. It could be that the kids are clones, or were once normal kids then "changed", or they're really all actually born from the Duchess in some kind of messed up ritual.
I personally think that the death of the Duke might have something to do with that.
The comments you're seeing (including this one) are just theories, that's it.
You made me so damn curious I started digging up spoilers on novel updates.
Lisette and Fjord are in fact different people, both tortured as science subjects by the Duchess.
Some kinda heavy spoilers:
The reason why our Empress remembers Lisette as problematic is because while Fjord simply listened to the Duchess, Lisette obeys her desperately and acts cruely.
She wants to be better than her brother because she is desperate for her "mother's" love, because she doesn't want to return to the basement. We'll be seeing her in future chapters from what I read.
At one point, I think sometime soon, she'll be introduced into this timeline as well, while Fjord is still alive. The Duchess will excuse her sudden appearance with the classic sickly-child-recovered-at-the-countryside trope.
Also, Lisette also has powers but unlike Fjord she's a puppeteer.
I was sure that Fjord was being forced to dress up as a girl and "kill" his identity to become Lisette (whatever the spelling was).
But now the comments are talking about other children, clones, incest etc. I'm actually really confused. Can anyone clarify or have spoilers?