@PRIMORDIAL THE ONE No she's his 14 year old blood related daughter. Her mother died giving birth to her and the pre-reincarnation memory buff MC got mentally/emotionally wrecked from becoming a Duke at a young age (like 20 or something) + losing his dearly beloved wife early too. As a result, he took it out on his innocent daughter and started his path towards evil as he obsessively looked for ways to revive Forsina's mother. He needed money, power, and resources for that of course, so that's how he ends up slain as the mid boss in the og game who takes advantage of the demon war chaos to try and usurp the throne.
I too am hoping that this is just platonic and the excitement of a young girl, finally receiving her father's affection for the first time. I'm hoping that it's just because she doesn't want to share his attention with a new mother, but wild that she brought up other girls her age as prospective wives. Then again, maybe it's not all that crazy considering how the king is and how young the queen looks 
But despite the main character constantly calling it out and putting a stop to it, if you don't like other people even bringing it up, you might wanna drop this story... And that's all I will say to avoid explicit spoilers 