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If we say

Who knows? - is OK.

But if we need to say

Whose brother knows it? or Whose brother does know it?
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TicceWhose brother knows it? or Whose brother does know it?
Hi Ticce

If whose brother is the subject, do/does/did is not used:
Whose brother knows it?

If another word is the subject, you need do:
Whose brother did you meet there?

CB
Comments  
TicceWhose brother knows it? or Whose brother does know it?
Whose brother knows it?
Students: Are you brave enough to let our tutors analyse your pronunciation?
 Cool Breeze's reply was promoted to an answer.
I agree with YL.