Since this is about sound, I agree with English Jon that an aural approach is best. However, for something visual, maybe you could get her to focus on the far ends of the keys, where the black ones separate the white ones unless the white ones are a semitone apart. If she's looking at those parts of the keys, then the interval between adjacent keys is a semitone and the interval if you skip a key is a whole tone, regardless of colors.

If you like this idea, you might even try covering the wide parts of the white keys with a strip of cardboard or cloth so she can ignore the fact that, say, C and D are misleadingly next to each other.

MMario, did that help you? If not, try this: a semitone is the interval between adjacent frets on a guitar, or between adjacent keys on a piano (if you don't skip over the black keys). Twelve semitones equals an octave, and we hear two notes an octave apart as "the same note only higher (or lower)".

For reasons having to do with history and with the way humans hear, we're used to a "diatonic" scale in which two semitones and five whole tones (each equal to two semitones) cover an octave. Piano keyboards, and our musical notation, are set up to make such scales easy. Thus the steps in the A minor scale, A-B-C-D-E-F-G-A, are whole, semi, whole, whole, semi, whole, whole. On a piano, you play that on the white keys.

The other five notes are written with sharps or flats; on the piano they're played with black keys.

Any "natural minor" scale has the same pattern of whole tones and semitones, but if you start on a different note you need to use black keys to get the pattern. For instance, the B minor scale goes B-C#-D-E-F#-G-A-B. Again it's whole, semi, whole, whole, semi, whole, whole.

The pattern of major scales is whole, whole, semi, whole, whole, whole, semi. The one that can be played on the white keys is C major: C-D-E-F-G-A-B-C.

These scales aren't carved in stone. You can hear intervals smaller than a semitone in music from India and in some 20th-century (and probably 21st-century) classical music and, I think, jazz. Anybody know about Scottish bagpipes?

Did that help?