Okay, if you're going to have both the main characters confess their love for each other, just have them date. The dating doesn't need to go well. There can be plenty of disfunction, conflict, and humor stemming from full blooded siblings dating. So tired of romance authors thinking, "Must end series the instant official dating starts." It's like they only know how to write stories leading up to dating, but can't write anything after that.
There's so much potential here. If the two siblings date, would they try to keep it a secret? You could have other characters start picking up on clues and speculating. Eventually people find out and we get their reactions. Then the two have to deal with the fallout. Advance the plot. If authors don't want to do that, then don't throw out mutually confirmations of love this early on. Otherwise, they end up having to twist the characters and plot into pretzels, just to avoid writing a story about them dating.