Even if the forum were as secure as Fort Knox, it wouldn't matter if your brother stole your key. Legally, if he does anything with your username and password, it is your responsibility. It may be only annoying on this forum, but what if he decides to steal your identity and buy a car with it? Or clean out your bank account?
So I don't think it's the forum's job to stop your hacker brother from stealing your password.
You could use Yubikey for a secure password. What it is is a hardware dongle that, when inserted, acts like a USB keyboard. When you physically push the device's button, it sends a complex alphanumeric password as if you were typing it. Since it looks to the operating system like a USB keyboard, it should work with any browser. The device isn't too expensive --- I purchased one for $25.
See
http://www.yubico.com/products/yubikey-hardware/yubikey/ for info
It won't stop him if he's doing a replay attack (i.e. capture your password and re-send it), but if he is typing in the password manually, this can make it nearly impossible for him. Yubikey can be told to send a password as long or complex as you'd like.
However, if the forum supports Yubikey's special add ons, Yubikey can be configured to send one-use passwords. This would probably even thwart your brother's replay attack.
If I were you, I would NEVER use any site if your hacker brother could see what you were doing. In a perfect world, not living with your hacker brother would be best, for the reasons I mentioned above. I'll state the blindingly obvious --- to the extent you have control over the matter, living with a criminal or criminal-in-training is a real bad idea.
