Try to explain yourself

"I may be your little sister, but I'm not naive." Emily says. "I know what this is." She holds the pink chastity cage in the palm of her hand. "Now how comes that my big brother has a chastity cage in his drawers?"

"It was just a stupid impulse buy. I regretted it right away and never used it" you say instinctively, trying to get her to drop the subject.
"Now John, there's nothing wrong with using toys to get yourself off. I should know" she says, smirking. You can feel your cock harden as she says that, your mind racing thinking about what that could mean. "So how come you never used it?" she asks. "Didn't find anyone who could hold the key?"

You stand there stunned. You have no idea what to say in response, not wanting to dig yourself deeper, and at the same time being amazed over how perverted the mind of your sister seems to be.

"This is what we are going to do John" Emily says. "I don't want you jerking off to your sister"
"I promise Em..." you blurt out, before being interrupted by your sister.
"I wasn't finished John. I clearly can't trust my perverted brother, so the only way to know for sure that you are not touching your cock to me, is to put this cage on, while I hold the key"

You freeze, not believing the words that comes out from your sisters mouth. Your sister wants you to put your chastity cage on. Your sister wants to be your keyholder.