If it was in the interest of keeping someone you love safe when you know they're in danger, then how far is too far?
For me, I don't think there is a point at which I would stop and say, "This is just too much. I must risk their lives."
Likewise, I'll say that what you did is justified in the interests of protecting your brother. However, it is now your responsibility to respect his privacy and not mention this to him or anyone else that knows him. Even if you know his deepest, darkest secret, as long as it doesn't threaten to hurt him, you need to let him keep on thinking his secret is safe.
Otherwise, you tried to protect him. Well done, let it go. Hopefully the less you dwell and worry about it, the more likely you are to forget his pornographic preferences.