From the Cornucopia rulebook: "When you play Young Witch, after you draw 2 cards and discard 2 cards, each other player may reveal a Bane card from his hand; if he does not, he gains a Curse."
"If Secret Chamber (from Dominion: Intrigue) is the Bane card, first you can reveal it for its Reaction ability, and then, if it's still in your hand, you can reveal it to avoid getting a Curse."
The way I read this, Grujah's comment is wrong; the Bane revealing phase would happen too late for you to reveal SC then use SC's ability to put it in your draw deck. I believe Reactions that happen on the play of an attack (like SC) occur when the card is played but before it's instructions are executed (which matters a lot for things like Pirate ship and Minion) -- but, as in the part above, the ability to reveal a Bane card clearly happens after Young Witch's executed instructions.