Re: If VI had featured Saavik instead of Valeris as the traitor
Either Kirk or McCoy should have been familiar enough with details of Chekov's family relationships to know whether there was a brother, rather than taking Sulu's word for it. Both of them must have reviewed his personal and/or medical files. Accepting Sulu's word as authority seems questionable, as this is the guy who thought himself one of the Three Musketeers, and went loopy over Landru.
Dr. Howard, Dr. Fine, Dr. Howard: For duty and humanity! --Men in Black (1934)