Well, this is @Sophia’s story; it all comes down to her and what she has in store for Marxus and what kind of things she has in mind if the Second Prince manages to develop a close relationship with him.
The two most vital things that come into my mind (personally) that can help the topic come up naturally is a stronger established bond between the brothers and some time passing by.
First, there would very much need to be a better developed relationship with between the Second Prince and Marxus. As you said, they haven’t really had many opportunities to develop a strong relationship with each other, very likely due to both of their individual parents. But as they get older, I feel that there’s only so much their parents can really do about keeping them apart and the two of them could have more moments with each other, to the point they have their own more solid opinions about each other (that don’t stem from their parents’ feelings). If the Second Prince’s relationship is low with him in general, I would never expect them to ever talk about the matter of course (though maybe it could be if Marxus feels like he needs to prove to the Second Prince that his feelings for Melissa are genuine). But if the Second Prince has been actively reaching out and trying to develop a stronger connection to him, it would feel really cruel of the First Prince not to meet him half-way with some amount of trust and vulnerability. A person can only stretch their hands so far when trying to connect with another and for only so long; the other party should be meeting them half-way after a while (maybe not right away, but at some point). I know if I was reaching out to someone and they weren’t even trying to meet me some parts of the way, I would feel like my desire to connect with the person must be one-sided and eventually just go my way once time passes too much and I become too tired to keep my hand reaching out to them.
Second, (as @IronRaptor pointed out), some time would need to pass by and there would need to be a good opportunity for the conversation to come up. I certainly can’t see them having the conversation too early in the story; I would feel like this type of conversation would be more likely to happen once things have cooled down a bit and the relationship between the characters comes to a point where the two are more comfortable around each other to bring up deeper conversations. Like one way I could see the conversation being brought up is if they have a good relationship with each other and the Second Prince has been offering advice and helping Marxus create a bridge to better his relationship with Melissa. I’m can’t really say if it goes with Marxus’ character, but maybe at some point, he turns the conversation to a more personal direction and finally directly asks the Second Prince ‘why have you been helping me’, for the first time showing a good deal of his true feelings toward the situation. Maybe even showing some semblance of guilt for the fact that he knew about the Second Prince’s feelings for Melissa and just pretended that it never existed, even as his little brother was reaching out and trying to help him understand Melissa better (again, this is just a guess, as I have no idea what is going on in Marxus’ mind, especially if the Second Prince had feelings for Melissa).
In many different ways. It’s why it can be really tricky on how to make a romance feel naturally developed and pace it in a interesting way.
Again, I will have to wait and see for myself if Andus really is good at managing a Kingdom and if he is, if that status stays that way. I keep imagining that if the Second Prince slowly rises in importance and beings to garner support that gives him a presence that is equal to that of Marxus, Andus will become increasingly…reckless and chaotic in his decisions (like a cornered animal thrashing violently in ever direction in a desperate attempt to take SOMETHING down with it, even if it is not the thing that it wanted to take down to begin with).
Other than that, I wonder now if Marxus might end up struggling between the respect he has for Andus (along with his longing for a more prominent presence from his father) and the (possible) familial love and emotional support he might possibly experience from the Second Prince. Or, if at the end of the day, he’ll always choose Andus over his little brother, despite (possibly) developing a powerful relationship with the young Prince and knowing how needlessly cruel Andus is toward the younger boy.