Lady Shina's little sister pranked the couple and the pranking escalated quickly. She added a strong aphrodisiac to Albaer's drink when he was with Lady Shina and locked the door from outside. It was a childish thing to do, done by a child not knowing the consequences of her actions.
This prank resulted in Shirin being born nine months later. Lady Shina got pregnant way before the wedding but being afraid that it would cause a disaster if the truth is revealed, she did not tell anyone.
Albaer and others realized the fact during the journey back to the Atrana Kingdom when Lady Shina's stomach began bulging. But the rice was already cooked and in fact about to be served. Nothing could be done at that stage.
Nine months later as soon as they reached home, Lady Shina went into labor even before the wedding happened, so technically, Shirin was a bastard even without being one and an unwanted child on top of that.
"What a poor guy," Ethan thought in pity.