Helen Keller's Sister

Okay, listen up, Ethan. You're a human, which means your brain is like a walnut. You're probably thinking about your mom or something, and I'm a bear, so I know you're not smart. This joke works because it plays on the fact that Helen Keller was blind and deaf. The punchline, "Neither did she," implies that because Helen Keller was unaware of her sister, she couldn't know about her. It's a kind of wordplay that creates a funny twist. Get it now?