What was the explicit purpose of Jesus' question to James and John? At Matthew 20:20-23, we read about  the request of the mother of James and John thus: 


  Then the mother of the sons of Zebedee came to him with her sons, and kneeling before him, she asked a favor of him.   And he said to her, “What do you want?” She said to him, “Declare that these two sons of mine will sit, one at your right hand and one at your left, in your kingdom.”   But Jesus answered, “You do not know what you are asking. Are you able to drink the cup that I am about to drink?” They said to him, “We are able.”   He said to them, “You will indeed drink my cup, but to sit at my right hand and at my left, this is not mine to grant, but it is for those for whom it has been prepared by my Father.”


Here,  Jesus is testing the commitment of the brothers to a life of suffering with and for Him . But, in spite of their having given a `Yes' answer, Jesus is non-committal about their place of choice in heaven . But then, what was His question to the brothers intended for?  


Are you able to drink the cup that I am about to drink?


What, if any, interpretations are offered by the Catholic Church on the explicit purpose of this question to James and John?   



I think my question needs a bit of elaboration.  To me, the answer - whatever it be- given by the disciples to the  challenge posed by Jesus , would not have  in any manner, influenced His  final reply  (that it is for The Father to decide) .   So, what was the need for Jesus to pose the challenge ?   Was He just making use of the opportunity to orient them to the difficult life that lay ahead ?
The most common take on that passage (and the related ones in Mark 10 and Matthew 10) is the difficulty in being a disciple (or in the case of those two, the immense difficulty and challenge of being an apostle).  

In the notes for the New American Bible hosted at the Vatican's web site, there is a cross reference to Matthew 10:38-40.   


  38  and whoever does not take up his cross and follow after me is not
  worthy of me.
  39 Whoever finds his life will lose it, and whoever loses his life for my sake will find it.
  40 "Whoever receives you receives me, and whoever receives me receives the one who sent me.  


The message or interpretation here isn't subtle, nor the purpose behind that cross reference.  To any disciple of Christ, be they the two He spoke to in that passage, or be it the verse speaking to someone receiving the Word through Scripture, the "cup he will be drinking" is an allusion to the cross, and he's indirectly asking them (or us) if they(we) can bear the cross and follow Him.  

I've heard this from the pulpit/ambo during the homily, during bible classes, and on retreats from clergy.  The theme and linkage is consistent.  (I get the idea that this is a core linkage taught in Seminary).   

The question interpreted for the laity in the present: can you really pick up that cross and follow in those footsteps?  (Message: it's very hard to do).