Maybe she gave him birth control or something
Spoiler:
Not this time, but she DOES do exactly that later on.
She gets thrown into nobles' jail after Etienne provides evidence of her and her family's crimes, and he visits her to basically say how shit of a mother she's been and that her ambitions are over. She gets PISSED and decides to screw him over one last time by calling a reporter, telling her that Etienne is lying to everyone b/c he wants to be emperor, and provides a heat-inducing drug so the reporter can see the truth for herself. So of course the reporter goes to chat with Etienne RIGHT BEFORE THE PARTY ANNOUNCING HIM AS CROWN PRINCE, gives him spiked tea, and gasp! It affects him! The empress was right, what a scoop!
...Except oops, the inducer interacts with Etienne's already-battered body and causes him excrutiating pain. The reporter starts to realize maaaaybe something's wrong and that's when Theodore conveniently shows up (totally at the empress' behest) to try and mark/rape Etienne. Thankfully, Rishar manages to save him, but the scene happened right outside the party doors and lots of ppl notice the omega pheromones. I think Theodore, the reporter, and the empress get charged with attempted murder (via poison) for the whole fiasco.
I hate the trope where they meet as children but this is chef kiss