Why which, not who?

In the example:

"And she's really belting along up there, a neat pass to Alicia Spinnet, a good find of Oliver Wood's, last year only a reserve -- back to Johnson and -- no, the Slytherins have taken the Quaffle, Slytherin Captain Marcus Flint gains the Quaffle and off he goes -- Flint flying like an eagle up there -- he's going to sc- no, stopped by an excellent move by Gryffindor Keeper Wood and the Gryffindors take the Quaffle - that's Chaser Katie Bell of Gryffindor there, nice dive around Flint, off up the field and -- OUCH -- that must have hurt, hit in the back of the head by a Bludger -- Quaffle taken by the Slytherins -- that's Adrian Pucey speeding off toward the goal posts, but he's blocked by a second Bludger -- sent his way by Fred or George Weasley, can't tell which -- nice play by the Gryffindor Beater, anyway, and Johnson back in possession of the Quaffle, a clear field ahead and off she goes -- she's really flying -- dodges a speeding Bludger -- the goal posts are ahead -- come on, now, Angelina -- Keeper Bletchley dives -- misses - GRYFFINDORS SCORE!"

I’ve heard relative pronoun whose antecedent is person is supposed to be who or that. Then why does the commentator say ‘which’ in his commentary?
(The sentence means, I can’t tell who did (for they’re twins) )