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The inconsistencies made some things come across as this.
In Sweet Valley High, Charlie Cashman is a racist bully with no redeeming qualities that we know of. In Sweet Valley Kids, he's not only a likeable troublemaker, he also has a Japanese friend named Kisho. Was it a Series Continuity Error or something much worse? The fact that Charlie's father was racist suggests that this could be the case.
By the time of Sweet Valley Twins, Charlie is showing more signs of becoming a jerk—his imitating the sexist homeroom teacher, etc. So yes, it's possible that like so many kids out there, he started out perfectly normal before being corrupted by his father.
In the beginning of Sweet Valley High, we meet Bruce's mother Marie who is a snobby bitch. In The Patmans of Sweet Valley, we meet a teenage Marie who is likeable, down to earth and possibly selfless to a fault. Did she change that much?
In a Sweet Valley Kids book, the twins learn that their cousin Kelly is worried that her parents are going to get a divorce. In the end she learns that they aren't and it's a Happy Ending. In a Sweet Valley High book, we learn that Kelly's mother is finally divorcing her father because he's been abusive and unfaithful. The ending to the Sweet Valley Kids book becomes very unsettling.
After Elizabeth fends off Bruce Patman, she asks him "Is this where all your stories come from?", referring to Bruce's playboy reputation. Making the reader wonder just how many girls Bruce has taken advantage of while they were intoxicated, or worse yet, outright raped if/when they put up a fight.