He had an affair, he was already married to empress. FL is emperor's third child. Lucy Pandora
Sorry for my ahove reply so " Read spoilers and apparently her mom is an elf and gave birth to her before the father got with the empress so she's older than the siblings. Her growth is due to the fact that she's half elf and human", I copied this from a previous comment
Did I miss something? If he married the empress after she was born, does that mean the two older siblings aren’t his children?