Are these sentences OK?
(friends, brothers etc.)
We're together until death.
Let's stay together until death.

And these;
If we are destined to win, then we will win.
You are destined to become a champion.
If we are destined to meet, we will meet again someday.

Question

Are these sentences OK?
(friends, brothers etc.)
   We're together until death.
   Let's stay together until death.
  
  And these;
   If we are destined to win, then we will win.
   You are destined to become a champion.
   If we are destined to meet, we will meet again someday.

Answer

We're together until death.  CORRECT GRAMMAR 
[WE ARE TOGETHER... SUGGESTS THE FUTURE TENSE = WE WILL BE TOGETHER UNTIL DEATH]
[THE FUTURE TENSE WOULD BE BETTER]

Let's stay together until death.  CORRECT GRAMMAR

If we are destined to win, then we will win.  CORRECT

You are destined to become a champion.   CORRECT

If we are destined to meet, we will meet again someday.  CORRECT 
[BUT "If we are destined to meet (implies/suggest meeting for the first time)]
["we will meet again" suggests meeting for one more time, which can only be true if the two people had already met at least once in the past]