The rule is to have your characters speak as they would given their situation, "irregardless" of the formal rules of grammar.
In the narrative, however, I would avoid it if it's redundant. I don't think "because otherwise" is redundant by itself, since the narrator (especially if in first person) could be explaining something. "She had to call her husband because otherwise he would drive to the airport not knowing she wouldn't be there." However, this could be "telly" if it is obvious to the reader already why a certain action was taken. Without knowing the entire context it's tough to say.
