The veracity of this allegation is something we don't really know about, sure. But it can't be denied that there are numerous so-called authors who publish 'autobiographies' of actors back then with sensationalized stories just to sell their books, and they are usually published when the actors are dead and gone and are unable to sue for defamation. This is exactly the case with the Brando Unzipped book, if you get it from there.
Nonetheless, I'm sorry that Pier was ever raped. If it's true that Brando raped her, my respect for him as a person will definitely diminish.