The hard part is over.
I'm matching you with one of our specialists who will be calling you in the next few minutes.
I am not a doctor but my guess is that your brother-in-law really is reacting to the death of his mother. The mentally handicapped can be extremely sensitive to situations and after all, this was probably the person who dedicated her life to keeping him at home. That's a big loss.
You are a very giving person to take this on and I wish you well. Look into the day care for some relief. Look up your state's Dept. of the Mentally Retarded and Develomental Disabilities. They should be able to point you to more local help.
Extra prayers for you!