I feel like the crown prince will get fucked, and the younger brother will get with the MC. Though tbh the duke seems like a much better fit... The 2nd prince is what? childhood friend who has been against her until not long ago. The duke? 1st time he meets her he is interested, 2nd time he pursues, 3rd time he is advancing. Just feel like the duke's interest is more "shallow" (barely know each other) but far more sincere.