An assassination arrangement. The King has never been fond of child but knows that child has a more legitimate claim on the throne than Marxus and knows that the Queen has ambitions to put her own child as king (ambitions that could bring harm to the only link that the king has left to the woman who truly had his heart). He offered something to his enemy to have him 'take care' of the prince without arousing, appealing to the other man/woman's greed. The man/woman, in turn, decides to have Sophia be the scapegoat in some way to make sure the prince if dealt with.
To make it even more unlikely that the Second Prince will become a possible candidate for the throne. Much like mentioned above, it is the Second Prince probably has the strongest claim to the throne and the Queen would probably have no qualms about harming the first prince to achieve this. So makes a arrangement with his enemy; arrange a marriage with his second son and he will offer something even more than peace to the greedy man/woman. The man/woman agrees and offers up Sophia as a scapegoat. Since their countries are long-time enemies, the Prince's favor becomes very low since no one in his country wants a Queen from enemy territory.
Looks at spoiler
...This makes me even more of a fan of Marxus and more against the idea of making things anymore awkward between him and Melissa. You have Arlan's undying loyalty, dear Brother; even if we can't truly have the love of our parents, we will have a brotherly bond that surpasses the supposed importance of a blood bond.
...Wow, I guess I better go see if the pigs have finally finished their jet packs. Who knew the old man was capable of developing beyond
Hilariously (more like bitter-sweetly), that's something that I could see Arlan doing something like that himself. Wanting to put things to closure between both of them so that they could start moving away from the 'what ifs' and move on to find happiness with another (maybe even being able to go back to their friendship), he'd do everything he could to find the Eadizlove flower and giving it to her (due to promising privately to himself to find it for her one day as a kid), explicitly stating that the flower is in honor of their friendship and how she will always be a dear friend of his, no matter the circumstances. Maybe he'd even try to find two of the flowers to offer to both his brother and childhood friend, to show his equal care for both of them.
...Although he would feel very hurt if Melissa does harm Sophia, questioning if she even valued their friendship.
You mean Team Marry-Them-All?
Out of curiosity, is it possible for the Second Prince to be so well liked and resourceful that, despite not being king himself, his voice seems to carry the same weight as Marxus during many matters? I imagine that the Queen would be somewhat please (though still hoping for a chance that the Second Prince will become the actual King with time) while King Andus will be very displeased and possibly privately worried to see that the Second Prince is beginning to seem as important to the others as King Marxus himself.