I play in a ukulele band and I was wondering what is the best whistle key to accompany a uke with re-entrant tuning in general?
Boy, that's nearly an impossible question because a uke might play in any key, or many keys even if you exclude the less likely ones (like C#minor etc.).
You really also need to specify which re-entrant tuning you mean, since both gCEA (C tuning) and aDF#B (D tuning) have their adherents, depending on which style and time period you're emulating. But taking a stab at it ...
For a C tuned uke, the most common easy I-VI-V major ukulele keys are probably C, F and G (corresponding to G, C and D fingerings on guitar). And for those keys, a C whistle gives you considerable mileage. The C whistle alone can actually cover all three of those keys. Adding a D whistle gives you easier G fingerings, and covers the next most common easy ukulele keys of D and A major (cf. guitar A and E).
The most common easy minor keys are probably Dm, Gm and Am (cf. guitar Am, Dm and Em). The C whistle covers Dm and Am, and Gm if you half-hole the scale 6th.
For a D tuned uke, raise everything above by a whole tone. So the common keys become D, G and A major, Em, Am and Bm - all covered by a D whistle. Adding an E whistle give you E and B major, and easier A major fingerings.
So as a rule of thumb: The whistle of the same key as the name of the uke tuning is a good starting point. C whistle for C tuning, D whistle for D tuning.
Depending on the kind of music you play, you may still find yourself half-holing and cross-fingering quite a bit. Popular music (new or old) can be fairly chromatic, use harmonic and melodic minor scales, modulate to odd keys, etc. But at least the C/D (or D/E) whistle pair gives you a reasonable diatonic starting point.
Hope that helps.