You were a child. A child cannot be held accountable for "sins of omission" any more than they should be held accountable for any other childish misdeed. Neither can your brother's mental state be laid at your feet for your childhood fearful inaction. If his mental health issues is in any part due to familial causes, the fault lays in the silence of the adults whose responsibility it was to speak.

However, I do understand your perspective on "terrible lies of silence." It is one of my maxims that, "Silence chooses too." If someone who can effect change chooses silence, they are culpable. For example, if someone watches a beating on the street and does nothing, doesn't even call the police, then that silent person is as guilty, in my estimation, as if they had rained blows on the victim. I also believe, though, that silence may be imposed, forced on the marginalized, the victimized, and/or those at risk – and that their silence is a conviction of the oppressor.