Ahahahah! Oh lordy, I can't stop laughing. It's probably not that bad though; he's probably more embarrassed than anything. How young is he? I was in the same situation, except I walked in on my brother (I was only about 11), and it just kinda blew over. But yeah, use the covers and lights off.
And yanno, at least you do it in the privacy of your own room. My little sister does it all damn day (and at least three times a day), in the living room, with my mother, my brother and myself all sitting two feet away from her watching TV. Best part is we've confronted her about it, and she denies it, and still does it.
You say you used a can of hairspray for a makeshift dildo...
Considering this, was it one of those thick 3 inch in diameter ones or a skinnier version?
As a tip for the next time you masturbate, do it in your bed under the covers with all the lights off so when someone comes in the room you can move around and look normal before they can turn the light on to see you.
Everyone got here before me and took all the jokes I was going to make. Anyway, my real response: with that description, if your brother tells, you could just say he didn't see quite right and you were simply changing clothes. Like all of them at once. And you needed the hairspray.