Hello Dear,
Don't get scared by what I'm saying but from what you have described, it appears that he is schizophrenic. The best you can do to your brother is to go to a psychiatrist and explain what you have said here. In the first meeting, you and your father only visit the doctor. He will guide about how to bring him there. [One trick would be to tell him that you are going to doctor for checkup instead of him]
To give a clear idea, here are few samples what a schizofrener does
Become overactive / underactive at times
Talk to themselves / over a phone when there is no actual call
Might think that somebody is acting illfully against her
Become angry/emotional
May speak something that may not be understood by us
May show signs of depression (Keeping still for hours, filled with sadness, etc)
Don't worry. Now there are good medicines available that will bring him back in life and away from such swings. But you have to do him a favour by taking him to psychiatrist as soon as possible. The more you delay, more time and medicines you will need for him to recover.
All the best