N.B: THIS APPLIES TO AND BASED HEAVILY ON THE LIGHT NOVEL, MANGA, AND ANIME, AND TAKES HEAVY INSPIRATION FROM THIS FANFIC I'VE PUT ABOVE
As we all know, Leon Fou Bartfort and Marie Fou Lafan were actually biological siblings in the real world and didn't know about it until much later on in the story. For the most part, the two were very much at odds with each other both in the otome game world as well as the real world.
So imagine this: The two started off as little kids who somehow managed to meet up with each other on a regular basis and somehow managed to become friends. During that time, they regained ALL their memories of the real world. Including those of the otome game. Also, they agree not to interfere with the in-game events too much in order to prevent, or at the very least prepare for, the war scenario. They even become genuine friends with Julius and his friends, Angelica, and Olivia, and advise them on life and relationships using their own experiences in the real world.
With this idea in mind, especially with it being heavily based on the above-mentioned fanfic, what would be the most realistic way for this scenario to seamlessly fit in and play out WITHOUT fundamentally altering the story too much?