Sharing is loving, as the saying goes, right? In such cases, you can simply demand it and your sibling can’t do anything about it. You guilt trip them over something you want to borrow, and you just decide to take it without their permission. Hey, you’re siblings; what one owns, you own too.
You may be somewhat unassuming to your sibling, but you clearly know more than you’re leading them on. When the time is right, and you’re alone together, you’re going to use their secret as blackmail. You’ll get them to do your homework and many other favors, just so you won’t tell their secret to anyone. You, my friend, have the upper hand.