This is probably way out of date and all resolved and stuff by now, but I couldn't help putting my two cents in!
You know most people don't just come out once. I have to "come out" to every new person I meet. I told my friends a long time before I told my parents. I told my brother before I told my parents, too, and then I told my mum before I told my Dad.
Maybe being honest with your brother won't be as bad as you think. Of course there's nothing wrong with keeping it to yourself until you're ready; but do you really want to spend the rest of your life lying? Telling your parents "oh, he's lying about the porn" is just going to set you up as enemies. Anyway, JaisonDrake is probably right; I doubt he'd mention anything to your folks... who talks about porn with their parents?!
Anyway, the point is, if your brother confronts you about it, maybe you could consider being honest. Coming out to him does not mean you will also have to tell your parents, or people at school. If he's genuine about being ok with it, then he will understand that you want to keep it secret.
Hope things all work out