I'm going to print reasonable requests (that seem patronising) in a communal bathroom shared with my brother
What kind of reasonable requests will you make?
I was going to place a sign on my brother's door that says "Don't take a piss during midnight or Pete will kick your ass" with a picture of a kicking leg.
You can be annoyed at what your brother's doing. But don't be passive aggressive about it and make a sign. Talk to him about it.
Fuck him. He's impossible to talk to.
You should still attempt to talk to him. Not doing so will just build resentment within both of you.