This makes zero sense, and is delivered in a silly tone. The government can even get information about you when it doesn't have warrants (good luck fighting it after the fact), or got warrants from a secret court. The government cannot get information about you which has not been collected, because that is impossible.
The point is that that information is already out there. It might not be on your laptop, but it exists.
Yeah they definitely need this data from your Mac to find out who your brother is.