"UNLESS YOU DRINK THE CUP" 

There's more to it than it sounds, the meaning is different from the way we understand it today. It was customary, it was the culture for the king or the master, or the Rabbi that when hosting a meal, on the right hand and on the left hand will be seated he favourites of the king. After the master or king had sipped the wine he would pass it on to the guest on his right-hand side, and he would drink likewise. The same with the one on the left hand side. BUT these two guests, when drinking the wine will surely not be saying: "Boy, what good wine, what vintage, is it Chardonnay, or....?" The drinking itself would mean a solemn declaration by the guest: "I promise to follow you, to follow your teachings, your beliefs, I am ready to give up my life for you. I have drunk and so I have committed myself." Both John and James knew the meaning of 'drinking from the chalice, and Jesus knew that they were both versed in the Jewish culture. That is why by the time 'drinking the chalice' became a symbol meaning accepting, embracing a particular belief even if it leads you to martyrdom. That is what Jesus had in mind. In fact James did suffer martyrdom and John, though he was not martyred, but he suffered much persecution, ultimately dying on the island of Pathmos, where he had written the 'Revelations'.