A D whistle plays the keys of both D and G quite easily, with no half-hole involved (key of A is also possible but involves a half-hole for the G#). Since the relative minors to D and G major are Bm and Em, respectively, those keys are also easily played, although Bm would be in the high register for most of the scale. The relative minor to Amaj would be F#m, playable with the same half-hole mentioned for A.