Ok, well this is how the joke goes:
Person A: My brother/sister and I know everything!
Person B: Really?
A: Yup!
B: Then what is (2 divided by 5) times the square root of 3?
A: 2 root 3 over 5.
B: Cool! Now what's the meaning of life?
A: My sibling is the one that knows that answer.
B: Aaaaah. Then what about...
And the conversation goes like that if the person knows the answer they answer the question, if they don't, well the sibling knows that answer. See how it works?