Wait, I’m confused. If her sister is already pregnant, how is ML the father? Since this ...

The sister are not pregnant with ML son. She pregnant with her husband daughter. In the past/webnovel the mc read ML had a son with her wife (the person MC will meet went she arrive at the castle)

Ahhhhh he’s not the male lead. He’s the villain’s father. He has a fiancé I think or something who he has a child with later, who becomes the villain. In the original her sister gives birth to her daughter in the other kingdom and her daughter and the villain grow up together. Her sister’s husband had died trying to get her back before their daughter was born. When they’re both ten years old the original female lead goes back to her own kingdom with her mother. Then this dudes son killed him soon after.

The original male lead was the son of the dude who was escorting her. The one who she said she was worried about his eye and leg? He’s a knight and his son also becomes a knight later.

Obviously now she’s the female lead and this dude is the male lead so who’s know what happens now. This is basically happening in what would be the very beginning of the original story.

From my understanding, he is the villain(who is yet to be born)'s father. The sister is pregnant with Brisney, the female lead of the original story, her father is another person entirely.

Her sister is pregnant with the female lead from the novel that happens 20 years in the future.
Erudian is the father of the villian who is in love (or at least obsessed) with bresley (FL) this villian we are talking about hadn't born yet.

The sister is married to the duke(?) of lebovny, that's the father.

Sister (princess of Lebovny) = FL's mom
Bro inlaw (duke of Lebovny) = FL's dad
Escort/knight (black haired) = ML's dad
That makes our Yerenica the FL's aunt, and the boss of ML's dad.

Meanwhile the Emperor = villain's dad

Basically the whole story is about the main characters' parents generation.

Ah, I see. Thank for the replies, everyone