Actually if you reread carefully the last episodes, if the step brother was not an evil human being (he poisoned his father, go after the restorer) and he was like just doing his job and co-op a bit with his half brother, his brother would not go after his position. He created the monster he was afraid of. And yes I agree, if your parent bring a kid with an affair partner you don't have to love them. But for sure you don't have to abuse them. He was on the will, he didn't have to either kill his father or keep sabotaging his brother. By doing nothing he would have everything.

Tbh i understand his step brother's feeling. Imagine your mother died from illness and your father bring his mistress with a son right after i didnt say what he did is valid, but i feel kinda bad