I have a question, is there a reason you have to live in the same house as your parents? You're 26, couldn't you get your own place and let your folks work it out between themselves? How old is your brother? If he's an adult, the same goes for him.
Your mom has a right to make her own choices. If she wants him to sleep at home, I think that's her business.
This way, you remove yourself from the situation and allow two adults to make their own decisions about how they live their lives. That is a highly ethical position to take--granting someone the dignity of making their own decisions about how they live. You don't have to like it, but you can respect it.
Are you getting treatment for your mental health issues?