I think it would probably be good to have a fourth resolution (so that the player is offered as many "good" solutions as "evil" ones,) where theĀ main character refuses to cooperate with their brother, exposes his plans, and stops him, but without successfully changing his mind or redeeming him. The ending in which you actually change his mind might be one that requires you to consistently uphold an honest and compassionate playstyle, where you make consistent efforts to help people, and don't betray people's trust or take advantage of others even when you can get away with it.
One element that I think would be interesting: throughout the game, the main character has the option to question various NPCs about their brother. He was well known, and numerous characters will report their experiences with him. Overwhelmingly, he's described as friendly, considerate, fair and impartial. But if the player actually tracks what activities and decisions he was involved in, they might notice that he always made choices which cemented his own power and social standing. Any faction whose support wasn't useful to him would end up getting the short end of the stick, even as they came away thinking of him as a fair and standup guy.
(I'm taking inspiration here from a person who I recently read describe their experiences in an online community, where they had a reputation for being fair, mature, and above petty politics and bickering, and as a result, attained much higher social standing than the people who others identified as being merely partisan. People from every faction would confide in him, and he would always hand down decisions that showed
understanding of both sides of the issue, because he was involved with people on every side. But while other members generally came away thinking of his decisions as fair, he really almost always sided with the socially powerful, whose support would preserve his social standing.)