Is Peter McNichol the man that plays Larry on television program NUMBERS Kristy McNichols brother?

You're close! Peter McNichol is not the actor who plays Larry on the show "NUMB3RS". That role is played by David Krumholtz.

However, Peter McNichol is related to Kristy McNichol. He is her older brother.

It seems like you're making a connection between the two actors and their last names. It's a common mistake to assume that actors with similar last names are related.