Honestly, this is a plot that's messier than a box of dropped tissues. It's not just the secret. You're carrying your closest friend's half-sibling, which automatically redraws every family line you ever knew. The baby becomes this living, breathing symbol of a betrayal she hasn't even consented to, and you're walking around with it. The emotional labor is staggering.
A lot of the tension hinges on power imbalance, too. Even if the dad is a decent guy, you're young and he's established, and now a baby ties you together forever. He has to navigate loving you, potentially losing his daughter's trust, and becoming a father again in a wildly unconventional way. The real conflict often isn't about whether they'll be together, but whether any of the relationships—yours with your friend, hers with her dad, the dad's with his future child—can survive the fallout. I've seen novels where the 'grovel' from the dad lasts years, not just a few chapters, because the damage is so deep and personal.
It works because it takes forbidden love and makes it devastatingly intimate. It's not a distant enemy, it's the person you text your daily memes to.