Why not say, Elle et ma sour? (It is 'specific').
Beth, this question (and similar) is covered in answers below the linked lesson. Worth reading through. In simple terms, as the lesson notes, ‘c’est’ is used when followed by an article/determiner - “c’est ma” in this case.
It's because you are speaking French and not English.. just the way the language is.. using English logic will cause you all sorts of problems.
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