Stewie tells Brian that changing the past can alternate the future, but back in "The Road to Germany" Stewie tells Brian that they can do what they like in the past.
Future Brian tells the 1999 version of himself to write the Harry Potter novels. However, the first novel in the series was already published in 1997 and would have already existed in 1999. Therefore, Brian could not have written them.
Brian: (on past Stewie's vocabulary) What, did you carry a thesaurus with you?
Brian from 1999: Whoa, crash ahoy.