I'm with
@arkvin and
@LayZzz on this one. Pun-time: I don't like the air around this heir.
It could be, that he wanted to join the race from the beginning. He played the innocent, but gifted, little fool-like brother, used his advantages (e.g. being overlooked/underestemated) and gathered information and placed his pawns. Then when he finally got close enough to that general (a guy that is a) hard to handle for people officially aiming for the throne and that b) will perish soon anyways, so nobody else bothered) and was on the peak of that guy's acknowledgement, he offed him to get the most out of his death. And also to have a "legit" reason to join the frail. "
I don't wanna, but now I have to."
The question, if his brother wouldn't like to become Emperor, becomes dubious from this POV, but I actually do believe he doesn't want to do harm to his twin brother. It's for the same reason why the MC didn't realize his bros real intention. It is genuine love. He wants MC to become the emperor in the end, because he believes him to be the best ruler. It's either that or (less interesting) he secretly despised his brother for being the free one, while he had to suffer from pressure and education, and that it is his brother's fault for not carrying his own share.
Nether the less I read some of those stories, in which the MC had this "big" secret. Turns out in most cases, that the other characters had the bigger ones. The smell of used >Plot device<.