Is there a good way to have a party of adventurers with only one player?

My sister expressed interest in playing D&D, and I’m interested in DMing, so I decided to try and run The Caves of Chaos from the D&D Next playtest packet. Reading through the materials, I became worried that my sister’s elf rogue wouldn’t last very long by herself. None of her friends are interested in D&D, so I’ve been toying with the idea of including other party members, either controlled by me or by her (The way Jason and Marcus play in FoxTrot).

This adventure isn’t the most serious thing in the world, obviously…just some Summer break fun. But do you guys see any reasons not to do this? Or, do you have any suggestions on how to run the adventure?