Wetting the cats thoroughly with water once or twice a week will keep the dander down and can help with allergies quite a bit but requires cooperation on the part of the cats. If your brother would consent to it, there is a chance that psychiatric care might help. I am not sure if any of this is advice you will find practical to use in this situation. Some people endanger their families and some people endanger themselves but a lot of people create situations in which their behaviors place someone at risk and often seem to be unwilling or unable to stop. It happens when people keep overly aggressive dogs, when they collect large numbers of cats and keep them in unhealthy situations and it happens when allergies or asthma produce a risk of death from a pet and people choose to keep them, anyway.
Allergies certainly contribute to chronic pulmonary disease.
Acute Asthma attacks certainly can kill. I am also aware of how difficult it is to change distructive behaviors. Perhaps the family can help your brother build an outside kennel for his cats. If the family pools its resources, the cost could be kept down. Michael Lebowitz, M.D., Professor of Medicine and Epidemiology at the University of Arizonia, Tucon, has done studies on the role of allergies and lung disease.