I am trying to decide if I should convince my little sister that she was adopted at the age of 12.
Why would you say such a thing to her?
I was just hoping to try and play a joke on her.
I understand you're just fooling around. However, you need to be very careful what you say to children about stuff like that. In fact, in general you shouldn't lie to kids because they're far too impressionable.
My sister is the dumbest person I know. She deserves this.
Lying to children is pretty mean, and saying that she is the dumbest person you know is as well. Nobody deserves to be lied to about being adopted.