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As a general rule, dialogue is not bound by the rules of grammar as tightly as the rest of the novel. Therefore, if a person says something a certain way, you write it that way. As far as your exam...
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#3: Attribution notice added
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#2: Initial revision
As a general rule, dialogue is not bound by the rules of grammar as tightly as the rest of the novel. Therefore, if a person says something a certain way, you write it that way. As far as your example goes, there is no right or wrong way to refer to Quorraline's mother in dialogue. If Quorraline refers to her both as 'mother' and 'mom,' then her dialogue should reflect that. As for the rest of the novel: Go with what is best for the book. If referring to Quorraline's mother solely as 'mother' sounds best, do it. If mixing the terms sounds better, do that. It is generally a good idea to refer to things (excluding during dialogue) the same way throughout a book. The more specific you get though, the less this rule of thumb applies. For example, if your book contains references to time, you should always write them the same way. Personal pronouns? Not so much. Now if you are trying to sound slightly more sophisticated, keeping it to 'mother' might be the way to go. It all depends on what you want to achieve with your novel.