You can do it if you're willing to give up your current game. If you save your Sim to the bin (from the dresser or mirror), you can use that Sim to create a new household in a new game. The Sim's traits and appearance will remain the same, but you won't have any of the relationships or skills. Then you just make a second household member and select for them to be siblings when you exit CAS. I don't know if this will accomplish what you want, but I can't think of another way to make it happen.