I am so sorry about the diagnosis. But hopefully there will be some palliative treatments (anti-nausea pills and diet, etc?) which you may find just affordable.
There certainly is some good advice on the Canine Kidney Failure group, which was started by Tony, one of our respected contributors here. Look further up the page of questions, as there is a thread about his group. I have just seen it.
You may find some tips and advice to extend the life of your little dog a bit further and make him as comfortable as possible for the time he has left, and I do hope so.
You are not alone, as many people here have/have had dogs who have renal failure.
As for his brother, and how he will adjust when your little man has to leave this world....well, he will certainly feel the loss. I have seen some animals accept totally and simply move on after the passing of a long term companion, and others who do certainly grieve.
Although, when that time comes you will be grieving yourself, the best idea is to provide lots of loving support, but also keep the daily routines, and time for play and walks as usual, so that they know some things haven't changed.
I had to cope with a grieving Border Collie (who wasn't my own dog but a neighbour's) -after the death of my girl. They had been incredibly close for many years.
I kept steady with him, and normal, but also let him know I was aware of his feelings. I visited every day, and it took him some time (a couple of weeks or more) before he was really ready to cheer up a little. Then he seemed to grow to accept. But he was an old dog and was never again quite as energetic as he had been up to that point. But the remainder of his life was pleasant enough.