First of all, I said earlier that you should tell him to get his own computer. I was wrong.
It's your parents issue to tell him to get his own computer.
When ancient people such as myself lived with their parents, the rule of thumb was, "when you're living under my roof, you'll live by my rules", period, no exceptions. Today's youth understand this concept as, "zero tolerance".
Part of solving a problem is recognizing that there is a problem. So, if mom and dad don't recognize a problem exists, you can sit around all day and post, think up clever ways to prevent him from using the computer, and bemoan your lot in life, pointlessly, all to no avail!
The answer was posted earlier and it's very simple, just password protect your and your parent's accounts, and don't open one for him.
Let your brother buy his own computer! Or, plan "B", whussy out, and buy him one. "I don't feel like buying one", is an unacceptable reason, and also an unacceptable reason to continue this thread.
Porn isn't illegal, nor necessarily harmfull, but it is one of those things, like cigarettes, that are socially unacceptable not to buy your own
People nowadays like to initiate a "reign of terror", with a fallback excuse that "I'm addicted and I can't help myself, so if you don't help me, I'll kill you, er I meant myself".