Hello Taka
Sorry, it wasn't very clear.
If a spectator (not one of the brothers) says 'who's going next?', there is a faint sense of 'openness': 'who (among those assembled here) is going next?'. Another spectator might answer, jokingly, "Haven't you heard? It's your turn next!"
Whereas if a spectator says 'which (one) is going next?', there's a presumption of 'restrictedness': to understand 'which', we already need to know the possible choices. The joke is no longer possible.
If the brothers are talking to each other, on the other hand, Wilbur can say 'which of us is going next?', because then the choice ('of us') is explicit. But he can't say simply 'which is going next?', because 'which' on its own would imply that the speaker is not included in the possible choices. So he has to use 'who'.
Or to put it another way: 'which' on its own seems to restrict the choice and exclude the speaker from that choice, in this context; while 'who' can mean 'any who'.
(I suspect I may have made it more complicated than it needs to be.)
MrP