the last sentence of a message I received:

olması imkansız kardeşim, tamam mı?


My try:
İts being is impossible, my brother, okay?

but it does not make sense in the context. Could it mean also:
İt cannot be, he is my brother, okay?

it cannot be, it is impossible brother, ok?

my try: Is it impossible my brother/sister, is that okay?
or It is impossible my brother/sister, okay?

maybe; My brother/sister, it is impossible, okay?
or?

Thank you both... but it still dosn't make sense, I am sure he wouldn't call me 'kardeşim'... that's why I was asking.
But nevermind, the general meaning is clear. Thank you.

I am not a native speaker but I will try to add something.

"(some subject) olması imkansız kardeşim, tamam mı?"
(Something's) happening is impossible, my brother, is that OK?

the "mı" after tamam makes it a question.

"kardeşim" can be used as expression of general closeness, but you say the person wouldn't refer to in that way, so I don't understand either. I seen it say in films to show an intensely, VERY, loyal relationship. There seem to be strong feeling here and you didn't give us enough to guess what they might involve.

Good luck with all this.