Responses

Yup, I'm thinking one of two things happened:
1) MC either did have the madness stuff going on but he didn't cause the murder, he was just framed and his madness was used as "proof".
Or
2) MC did not have madness and that was fabricated by the siblings. They just manipulated things enough to create a situation were they claimed the MC was mad.
The brother is shit. Point blank, can't say anything else about him. Obviously he did something that the MC knew and they want to ensure his memories don't come back. Makes me think that MC wasn't the one who committed his sin, and perhaps it was the brother and the MC was the only one to see it. Then everything else has been manipulation by the siblings.
The sister is arguably equally as bad as the brother. She is the true master of the house, she has the authority to ensure her brother does not hurt the MC. She could have MC sleep in better quarters, she could have MC serve her directly instead, or she could take MC with her on her trips outside the castle. However, she doesn't and she allows her brother to hurt MC with no consequences. But she not only pretends to not see, she also supports whatever her brother did and has made sure that the MCs memories do not come back. It seems she also made a deal with the ML but she hasn't kept her word either.
Both siblings are shit.