Well, you can't really say any scale doesn't exist, because it's always possible to build it. But your point that it would rarely if ever (never say never!
) be used in real life is quite right. Western music theory does in fact allow for a D# major scale (or weirder ones than that); Western musical practice, however, doesn't as a rule. But you're didn't really address my point, which is that major and relative minor, in every case without exception, are by definition a minor 3rd apart, not an augmented 2nd or any other interval.If it makes things a little clearer, let's try another example. Take the key of Gb and its relative minor, Eb minor. The reason why, in this context, it's slightly better to say they're a minor 3rd apart than to call the distance three semitones (although the latter is of course correct as far as it goes) is that three semitones could also describe the difference between Eb minor and F# major; however, Eb minor is not and, by definition, cannot be the relative minor of F# major. And as you know, all of these keys exist in the real world. If you merely say the distance is three semitones, you fail to specify why Eb minor can't be the relative minor of F# major. After all, they're three semitones apart, aren't they? It simply makes no sense to say that three semitones is a MORE correct way of explaining the interval between major and relative minor.
You can make the same point for Db major/C# major and Bb minor, all of which keys are used in practice.
Nothing of what you said contradicts what I said, which is why it's perfectly OK and in fact slightly preferable to refer to that interval as a minor 3rd.
